A book series is a sequence of books having certain characteristics in common that are formally identified together as a group. Book series can be organized in different ways, such as written by the same author, or marketed as a group by their publisher.
Fiction books 
Fictional series typically share a common setting, story arc, set of characters or timeline. They are common in genre fiction, particularly crime fiction, men's adventure and science fiction, as well as in children's literature.
Some works in a series can stand alone—they can be read in any order, as each book makes few, if any reference to past events, and the characters seldom, if ever, change. Many of these series books may be published in a numbered series. Examples of such series are works like The Hardy Boys, Nancy Drew, and Nick Carter.
Some series do have their characters go through changes, and make references to past events. Typically such series are published in the order of their internal chronology, so that the next book published follows the previous book. How much these changes matter will vary from series to series (and reader to reader). For some, it may be minor—characters might get engaged, change jobs, etc., but it does not affect the main storyline. Examples of this type include Tony Hillerman's Jim Chee and Joe Leaphorn books. In other series, the changes are major and the books must be read in order to be fully enjoyed. Examples of this type include the Harry Potter series.
There are some book series that are not really proper series, but more of a single work so large that it must be published over two or more books. Examples of this type include The Lord of the Rings volumes or the Dark Tower series by Stephen King.
Some authors make it difficult to list their books in a numerical order when they do not release each work in its 'proper' order by the story's internal chronology. They might 'jump' back in time to early adventures of the characters, writing works that must be placed before or between previously published works. Thus, the books in a series are sometimes enumerated according to the internal chronology rather than in publication order, depending on the intended purpose for the list. Examples of this series include works from the Chronicles of Narnia, where the fifth book published, The Horse and His Boy, is actually set during the time of the first book, and the sixth book published, The Magician's Nephew is actually set long before the first book. This was done intentionally by C. S. Lewis, a medieval literature scholar. Medieval literature did not always tell a story chronologically.
Academic and scholarly publication 
In scholarly and academic publishing, scientific and non-fiction books that are released serially (in successive parts) once a year, or less often, are also called a series. (Publications that are released more often than once a year are known as periodicals.) The connection among books belonging to such a series can be by discipline, focus, approach, type of work, or geographic location. Examples of such series include Antwerp working papers in linguistics; Early English manuscripts in facsimile; Garland reference library; Canterbury Tales Project; Early English Text Society.
See also