EPUB

For other uses, see Epub (disambiguation).

Electronic Publication (EPUB)

Filename extension	.epub
Internet media type	application/epub+zip (unofficial)
Developed by	International Digital Publishing Forum (IDPF)
Initial release	September 2007
Latest release	3.0 October 11, 2011
Type of format	e-book file format
Contained by	OEBPS Container Format (OCF) (ZIP)
Extended from	Open eBook, XHTML, CSS, DTBook
Website	IDPF Home Page

EPUB (short for electronic publication; alternatively capitalized as ePub, ePUB, EPub, or epub, with "EPUB" preferred by the vendor) is a free and open e-book standard by the International Digital Publishing Forum (IDPF). Files have the extension .epub.

EPUB is designed for reflowable content, meaning that the text display can be optimized for the particular display device used by the reader of the EPUB-formatted book, although EPUB now also supports fixed-layout content. The format is meant to function as a single format that publishers and conversion houses can use in-house, as well as for distribution and sale. It supersedes the Open eBook standard.

History

EPUB became an official standard of the International Digital Publishing Forum (IDPF) in September 2007, superseding the older Open eBook standard.

In August 2009, the IDPF announced that they would begin work on maintenance tasks of the EPUB standard. Two broad objectives were defined by this working group: "One set of activities governs maintenance of the current EPUB Standards (i.e. OCF, OPF, and OPS), while another set of activities addresses the need to keep the Standards current and up-to-date." The working group was expected to be active through 2010, publishing updated standards throughout its lifetime. On April 6, 2010, it was announced that this working group would complete their update in April 2010. The result was to be a minor revision to EPUB 2.0.1 which "corrects errors and inconsistencies and does not change functionality". On July 2, 2010, drafts of the version 2.0.1 standards appeared on the IDPF website.

On April 6, 2010, it was announced that a working group would be formed to revise the EPUB specification. In the working group's charter draft, 14 main problems with EPUB are identified which the group will address. The group was chartered through May 2011, and was scheduled to submit a final draft on May 15, 2011. An initial Editors Draft for EPUB3 was published on November 12, 2010, and the first public draft was published on February 15, 2011. On May 23, 2011, the IDPF released its proposed specification for final review. On October 10, 2011, the IDPF announced that its membership had approved EPUB 3 as a final Recommended Specification.

Features

• Free and open
• Reflowable (word wrap) and resizable text
• Inline raster and vector images
• Embedded metadata
• DRM support
• CSS styling
• Support for alternative renditions in the same file
• Use of out-of-line and inline XML islands to extend the functionality of EPUB

File format

Version 3.0 (current version)

EPUB 3 consists of a set of four specifications:

• EPUB Publications 3.0, which defines publication-level semantics and overarching conformance requirements for EPUB Publications.
• EPUB Content Documents 3.0, which defines profiles of XHTML, SVG and CSS for use in the context of EPUB Publications.
• EPUB Open Container Format (OCF) 3.0, which defines a file format and processing model for encapsulating a set of related resources into a single-file (ZIP) EPUB Container.
• EPUB Media Overlays 3.0, which defines a format and a processing model for synchronization of text and audio.

Detailed descriptions of the differences between 3.0 and 2.0.1 can be found on the IDPF website.

The EPUB 3.0 format is intended to address the following criticisms:

• While good for text-centric books, EPUB may be unsuitable for publications which require precise layout or specialized formatting, such as a comic book.

• A major issue hindering the use of EPUB for most technical publications is the lack of support for equations formatted as MathML. They are currently included as bitmap or SVG images, precluding proper handling by screen readers and interaction with computer algebra systems. Support for MathML is included in the EPUB 3.0 specification.

• Another criticism of EPUB revolves around the specification's lack of detail on linking into, between, or within an EPUB book, as well as its lack of a specification for annotation. Such linking is hindered by the use of a ZIP file as the container for EPUB. Furthermore, it is unclear if it would be better to link by using EPUB's internal structural markup (the OPF specification mentioned above) or directly to files through the ZIP's file structure. The lack of a standardized way to annotate EPUB books could lead to difficulty sharing and transferring annotations and therefore limit the use scenarios of EPUB, particularly in educational settings, because it cannot provide a level of interactivity comparable to the web.

Version 2.0.1

EPUB version 2.0.1 consists of three specifications:

• Open Publication Structure (OPS) 2.0.1, contains the formatting of its content.
• Open Packaging Format (OPF) 2.0.1, describes the structure of the .epub file in XML.
• Open Container Format (OCF) 2.0.1, collects all files as a ZIP archive.

EPUB internally uses XHTML or DTBook (an XML standard provided by the DAISY Consortium) to represent the text and structure of the content document, and a subset of CSS to provide layout and formatting. XML is used to create the document manifest, table of contents, and EPUB metadata. Finally, the files are bundled in a zip file as a packaging format.

Open Publication Structure 2.0.1

An EPUB file uses XHTML 1.1 (or DTBook) to construct the content of a book as of version 2.0.1. This is different from previous versions (OEBPS 1.2 and earlier) which used a subset drawn from XHTML. There are, however, a few restrictions on certain elements. The mimetype for XHTML documents in EPUB is application/xhtml+xml. For a table of the required XHTML modules and a description of the restrictions, see Section 2.2 of the specification.

Styling and layout are performed using a subset of CSS 2.0, referred to as OPS Style Sheets. This specialized syntax requires only a portion of CSS properties to be supported by reading systems and adds a few custom ones. Custom properties include oeb-page-head, oeb-page-foot, and oeb-column-number. Font-embedding can be accomplished using the @font-face property, as well as including the font file in the OPF's manifest (see below). The mimetype for CSS documents in EPUB is text/css. For a table of supported properties and detailed information, see Section 3.0 of the specification.

EPUB also requires that PNG, JPEG, GIF, and SVG images be supported using the mimetypes image/png, image/jpeg, image/gif, image/svg+xml. Other media types are allowed, but creators must include alternative renditions using supported types. For a table of all required mimetypes, see Section 1.3.7 of the specification.

Unicode is required, and content producers must use either UTF-8 or UTF-16 encoding. This is to support international and multilingual books. However, reading systems are not required to provide the fonts necessary to display every unicode character, though they are required to display at least a placeholder for characters that cannot be displayed fully.

An example skeleton of an XHTML file for EPUB looks like this:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
  <head>
    <meta http-equiv="Content-Type" content="application/xhtml+xml; charset=utf-8" />
    <title>Pride and Prejudice</title>
    <link rel="stylesheet" href="css/main.css" type="text/css" />
  </head>
  <body>
    ...
  </body>
</html>

Open Packaging Format 2.0.1

The OPF specification's purpose is to "[define] the mechanism by which the various components of an OPS publication are tied together and provides additional structure and semantics to the electronic publication." This is accomplished by two XML files with the extensions .opf and .ncx.

.opf file

The OPF file, traditionally named content.opf houses the EPUB book's metadata, file manifest, and linear reading order. This file has a root element package and four child elements: metadata, manifest, spine, and guide. All of these except guide are required. Furthermore, the package node must have the unique-identifier attribute. The .opf file's mimetype is application/oebps-package+xml.

The metadata element contains all the metadata information for a particular EPUB file. Three metadata tags are required (though many more are available): title, language, and identifier. title contains the title of the book, language contains the language of the book's contents in RFC 3066 format or its successors, such as the newer RFC 4646 and identifier contains a unique identifier for the book, such as its ISBN or a URL. The identifier's id attribute should equal the unique-identifier attribute from the package element. For a full listing of EPUB metadata, see Section 2.2 of the specification.

The manifest element lists all the files contained in the package. Each file is represented by an item element, and has the attributes id, href, media-type. All XHTML (content documents), stylesheets, images or other media, embedded fonts, and the NCX file should be listed here. Only the .opf file itself, the container.xml, and the mimetype files should not be included. Note that in the example below, an arbitrary media-type is given to the included font file, even though no mimetype exists for fonts.

The spine element lists all the XHTML content documents in their linear reading order. Also, any content document that can be reached through linking or the table of contents must be listed as well. The toc attribute of spine must contain the id of the NCX file listed in the manifest. Each itemref element's idref is set to the id of its respective content document.

The guide element is an optional element for the purpose of identifying fundamental structural components of the book. Each reference element has the attributes type, title, href. Files referenced in href must be listed in the manifest, and are allowed to have an element identifier (e.g. #figures in the example). A list of possible values for type can be found in Section 2.6 of the specification.

An example OPF file:

<?xml version="1.0"?>
<package version="2.0" xmlns="http://www.idpf.org/2007/opf" unique-identifier="BookId">

  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:opf="http://www.idpf.org/2007/opf">
    <dc:title>Pride and Prejudice</dc:title>
    <dc:language>en</dc:language>
    <dc:identifier id="BookId" opf:scheme="ISBN">123456789X</dc:identifier>
    <dc:creator opf:file-as="Austen, Jane" opf:role="aut">Jane Austen</dc:creator>
  </metadata>

  <manifest>
    <item id="chapter1" href="chapter1.xhtml" media-type="application/xhtml+xml"/>
    <item id="stylesheet" href="style.css" media-type="text/css"/>
    <item id="ch1-pic" href="ch1-pic.png" media-type="image/png"/>
    <item id="myfont" href="css/myfont.otf" media-type="application/x-font-opentype"/>
    <item id="ncx" href="toc.ncx" media-type="application/x-dtbncx+xml"/>
  </manifest>

  <spine toc="ncx">
    <itemref idref="chapter1" />
  </spine>

  <guide>
    <reference type="loi" title="List Of Illustrations" href="appendix.html#figures" />
  </guide>

</package>

.ncx file

The NCX file (Navigation Control file for XML), traditionally named toc.ncx, contains the hierarchical table of contents for the EPUB file. The specification for NCX was developed for Digital Talking Book (DTB), is maintained by the DAISY Consortium, and is not a part of the EPUB specification. The NCX file has a mimetype of application/x-dtbncx+xml.

Of note here is that the values for the docTitle, docAuthor, and meta name="dtb:uid" elements should match their analogs in the OPF file. Also, the meta name="dtb:depth" element is set equal to the depth of the navMap element. navPoint elements can be nested to create a hierarchical table of contents. navLabel's content is the text that will appear in the table of contents generated by reading systems that use the .ncx. navPoint's content element points to a content document listed in the manifest and can also include an element identifier (e.g. #section1).

A description of certain exceptions to the NCX specification as used in EPUB can be found in Section 2.4.1 of the specification. The complete specification for NCX can be found in Section 8 of the Specifications for the Digital Talking Book.

An example .ncx file:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE ncx PUBLIC "-//NISO//DTD ncx 2005-1//EN"
"http://www.daisy.org/z3986/2005/ncx-2005-1.dtd">

<ncx version="2005-1" xml:lang="en" xmlns="http://www.daisy.org/z3986/2005/ncx/">

  <head>
<!-- The following four metadata items are required for all NCX documents,
including those conforming to the relaxed constraints of OPS 2.0 -->

    <meta name="dtb:uid" content="123456789X"/> <!-- same as in .opf -->
    <meta name="dtb:depth" content="1"/> <!-- 1 or higher -->
    <meta name="dtb:totalPageCount" content="0"/> <!-- must be 0 -->
    <meta name="dtb:maxPageNumber" content="0"/> <!-- must be 0 -->
  </head>

  <docTitle>
    <text>Pride and Prejudice</text>
  </docTitle>

  <docAuthor>
    <text>Austen, Jane</text>
  </docAuthor>

  <navMap>
    <navPoint class="chapter" id="chapter1" playOrder="1">
      <navLabel><text>Chapter 1</text></navLabel>
      <content src="chapter1.xhtml"/>
    </navPoint>
  </navMap>

</ncx>

Open Container Format 2.0.1

An EPUB file is a group of files conforming to the OPS/OPF standards that is wrapped in a ZIP file. The OCF specifies how these files should be organized in the ZIP, and defines two additional files that must be included.

The mimetype file must be a text document in ASCII and must contain the string application/epub+zip. It must also be uncompressed, unencrypted, and the first file in the ZIP archive. The purpose of this file is to provide a more reliable way for applications to identify the mimetype of the file than just the .epub extension.

Also, there must be a folder named META-INF which contains the required file container.xml. This XML file points to the file defining the contents of the book. This will be the OPF file, though additional alternative rootfile elements are allowed.

Apart from mimetype and META-INF/container.xml, the other files (OPF, NCX, XHTML, CSS and images files) are traditionally put in a directory named OEBPS.

An example file structure:

--ZIP Container--
mimetype
META-INF/
  container.xml
OEBPS/
  content.opf
  chapter1.xhtml
  ch1-pic.png
  css/
    style.css
    myfont.otf

An example container.xml, given the above file structure:

<?xml version="1.0" encoding="UTF-8" ?>
<container version="1.0" xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
  <rootfiles>
    <rootfile full-path="OEBPS/content.opf" media-type="application/oebps-package+xml"/>
  </rootfiles>
</container>

Digital rights management

An EPUB file can optionally contain DRM as an additional layer, but it is not required by the specifications. In addition, the specification does not name any particular DRM system to use, so publishers can choose a DRM scheme to their liking. However, future versions of EPUB (specifically OCF) may specify a format for DRM.

The EPUB specification does not enforce or suggest a particular DRM scheme. This could affect the level of support for various DRM systems on devices and the portability of purchased e-books. Consequently, such DRM incompatibility may segment the EPUB format along the lines of DRM systems, undermining the advantages of a single standard format and confusing the consumer.

When present, DRMed EPUB files must contain a file called rights.xml within the META-INF directory at the root level of the ZIP container.

Validation

An open source tool called epubcheck exists for validating and detecting errors in the structural markup (OPS, OPF, OCF) as well as the XHTML and image files. The tool can be run from the command line, or used in webapps and applications as a library. A large part of the original work on the tool was done at Adobe Systems.

Software

Software reading systems

Software that reads, and presumably displays, EPUB files is called a reading system. An EPUB reading system is defined as:

“A combination of hardware and/or software that accepts OPS Publications and makes them available to consumers of content. Great variety is possible in the architecture of Reading Systems. A Reading System may be implemented entirely on one device, or it may be split among several computers....”

Reading Systems and Software

Software	Platform	DRM formats supported	Notes
Adobe Digital Editions	Windows, Mac OS X	Adobe Content Server	Requires online activation.
Aldiko	Android	Adobe Content Server	Supports ePub for Android devices.
AZARDI	Windows, Mac OS X, GNU/Linux	?	Supports ePub 3, ePub 2 and the secure AZD format.
BookGlutton	Web	?	Free online ePub reader focusing on the social aspects of reading.
calibre	Windows, Mac OS X, GNU/Linux	None	Primarily for library management, conversion, and transferring to devices, it includes a reader. "Calibre: About".
CoolReader	Windows, GNU/Linux, Android	None	XML/CSS based E-Book reader for desktops and handheld devices. Supported formats: FB2, TXT, RTF, TCR, HTML, EPUB, CHM. Has GUI implementation for E Ink base devices. Most popular SourceForge epub application.
Dorian	Symbian	?	Free ePub reader.
EPUBReader	Firefox add-on	None	Enables reading ePub-files from within Firefox.
FBReader	Windows, GNU/Linux, PDAs	None	Incomplete ePub support.
FBReaderJ	Android	?	Open source.
Google Books	Web application, Android, iOS	?	Supports downloading purchased books as ePub and/or PDF.
iBooks	iOS	FairPlay	Supports EPUB 2 and EPUB 3. Books not readable directly on computers (Mac or PC) yet.
IkuReader	Nintendo DS	?	EPUB, FB2 support.
Kitabu	Mac OS X	None	Free lightweight ePub reader. Simple library management.
Lexcycle Stanza	iOS, Windows, Mac OS X	Yes	Acquired by Amazon in 2009.
Mobipocket	Windows, BlackBerry, Symbian, Windows Mobile	None	Converts EPUB into .PRC on import.
Moon+ Reader	Android	?
NOOK for Mac	Mac OS X	?	Barnes & Noble account required.
Okular	KDE Platform	?
PageTurner Reader	Android	None	Open Source (GPL) ePub Reader App with built-in synchronisation. Uses epublib.
readMe	iOS	?	EPUB, FB2 and PDF support.
sReader	iOS	?	EPUB support.
Stanzetta	Windows Phone	None	Supports ePub on Windows Phones.

Editing systems

Creation Software

Software	Platform	Notes
ABBYY FineReader	Windows	Commercial license. Version 11 exports to EPUB format.
Adobe InDesign	Windows, Mac OS X	Commercial license. Exports to EPUB format. Note that versions prior to 5.5 create EPUBs that require significant editing in order to pass ePubCheck or ePubPreFlight. Plan on using Sigil or studying Liz Castro's EPub Straight to the Point book if you want to make EPUBs using Indesign 5.0.
Adobe RoboHelp	Windows	Online documentation tool that supports export to EPUB format
Apache OpenOffice 3.4	Windows, Mac OS X, GNU/Linux	?	EPUB creation.
Aquafadas Digital Publishing System	Windows, Mac OS X	Commercial license. Exports to epub3 Fixed Layout format since version 2.0. The publishing system proposes a rich toolset of enrichments for the InDesign documents. The following subset is preserved during the epub3 export : videos, slideshows, sounds, xhtml snippets, links, actions. The generated epub preserves the layout and styles by embedding the appropriate fonts. It does not require any manual editing to pass the epubcheck test.
Atlantis Word Processor	Windows, Portable app	Converts any document to EPUB; supports multilevel TOCs, font embedding, and batch conversion. Shareware.
Booktype	Web	Book production platform which outputs to many formats including ePub. Free Software under the GPL license. The platform also enables importing of content in various formats and collaborative editing.
calibre	Windows, Mac OS X, GNU/Linux	Conversion software and e-book organizer. Free Software under the GPL license. Allows plugins, including for editing EPUB files; there is for instance a plugin to merge several EPUB files into one.
eLML	Windows, Mac OS X, GNU/Linux	The eLesson Markup Language is a platform-independent XML-based open source framework to create eLearning content. It supports various output formats like SCORM, HTML, PDF and also eBooks based on the ePub format.
Feedbooks	Web	Free cloud service for downloading public domain works and for self-publishing.
Folium Book Studio	Web	Convert Microsoft Word and other documents to the EPUB 3 format and convert manuscripts to typographically corrected proofs. Edit online, design cover art or upload from another source. Drop in images and align with text. Preview and export the EPUB or HTML for direct distribution to Amazon, Apple, Barnes & Noble, Lulu, or others.
Help & Manual	Windows	Single source publishing tool that generates ePUB amongst several other documentation formats. Commercial license.
HelpNDoc	Windows	Help authoring tool which can generate EPUB files amongst other formats. Free for personal use, commercial otherwise.
iBooks Author	Mac OS X	Desktop publishing and page layout application. Free from Apple. Can export .ibooks format, which is a proprietary format based on EPUB. There are restrictions on the commercial distribution of works created with iBooks in the .ibooks format. These restrictions apply to the .ibooks format only and it can be argued that a file renamed to .epub is not distributed in the .ibooks format.
iStudio Publisher	Mac OS X	Desktop publishing and page layout application. Commercial license.
Jutoh	Windows, Mac OS X, Linux	WYSIWYG ebook editor-compiler. Exports to both ePUB and Mobipocket (Kindle) formats. Commercial license.
Lulu.com	Web	Upload and convert .doc, .docx, or PDF manuscripts to an ePub. Then choose a title, create a cover, describe your ePub, and pick a price. It's free to publish and sell.
Madcap Flare	Windows	Single source publishing tool that allows content to be exported in the ePUB format. Commercial license.
oXygen XML Editor	Mac OS X, Windows, Linux	XML Editor is the first tool which offers support for creating, transforming and validating documents composing the EPUB package. Commercial license.
Pandoc	GNU/Linux, Mac OS X, Windows	Universal converter between many formats (even TeX -> EPUB). Free Software under the GPL license.
Pages	Mac OS X	Word processor (part of the iWork '09 suite) that can export to EPUB format (Pages '09 only, and only with the iWork 9.0.4 update).
QuarkXPress	Mac OS X, Windows	Desktop Publishing Tool, Page Layout Application. Exports also to the ePUB format. Commercial license.
Serif PagePlus X6	Windows	Desktop Publishing Program. Exports also to the ePUB format. Commercial license.
Scrivener	Windows, Mac OS X	Commercial program for writers. Includes organization capabilities for fiction writers. Publishes to multiple formats.
Sigil	Windows, GNU/Linux, Mac OS X	Free, Open source under GPLv3. Currently the only application that can also open and edit EPUB books, instead of just converting from other formats to EPUB. Does not currently support embedding video or audio in EPUB.

Hardware reading systems

The boundary between hardware and software is not clear cut. Some of these devices are dedicated to e-book tasks while others are platforms that include e-book readers or can have them added. See "Comparison of e-book readers" for details of dedicated devices (not all support EPUB).

• Android devices (using Aldiko, CoolReader, FBReader, and so on)
• Barnes & Noble Nook
• BeBook
• Bookeen Cybook Gen3, Cybook Opus
• Boox
• COOL-ER
• Cruz (Velocity Micro)
• eClicto
• ECTACO jetBook and ECTACO jetBook Lite
• eSlick
• GNU/Linux tablets and PDAs such as Sharp Zaurus and Nokia 770, n800, n810, and n900
• Hanlin eReader
• iPad, iPhone, and iPod Touch (Lexcycle Stanza or iBooks on iOS 3.2+)
• iRex Digital Reader 800 and 1000
• iriver Story
• Kobo eReader
• Plastic Logic
• PocketBook Reader
• Sony Reader

See also

• Comparison of e-book formats
• Open Packaging Convention
• XML Paper Specification

External links

• EPUB Wiki/Home Page includes issue and bug tracking.
• EPUB Format Construction Guide (also available in EPUB)
• Video: Format overview of the .epub file
• Publisher Tim O'Reilly explains the importance of EPUB
• Build a digital book with EPUB from IBM developerWorks
• EPUB tooling in the Eclipse project