A new feature, the collection extension (also called "wiki-to-print" or the book tool) has been enabled on six more Wikipedias (French, Polish, Dutch, Portuguese, Spanish, and Simple English) and on the English Wikipedia for logged-in users, according to a post on the Wikimedia blog. The extension had previously been put into production on the German-language Wikipedia  and on Wikibooks and Wikisource.
The new extension means that for logged-in users on the English Wikipedia, there are three new links in the left-hand sidebar. The link in the toolbox labelled "PDF version" renders a pdf version on the fly of the article currently being viewed. The next two new links are in a new box called "create a book." These links enable creating collections of articles, or "books." The link "Add wiki page" adds the page currently being viewed to a new book, while "Books help" links to the help page with directions for producing books. Books that are created may be downloaded in pdf format or OpenDocument (.odf) format by clicking on the "show book" link once all the desired pages are added. This link leads to the Special:Book page, where a book title may be added and the order of pages changed. Books may also be shared with others, and there is a page to list books that are created at Wikipedia:Books.
Books can also be printed out for a fee by sending them to PediaPress, the developers of the new extension, which will create a printed book for any book produced using the extension. You can also go to the PediaPress collections site and develop a collection of content from any MediaWiki-based public wiki to be printed. Books printed by PediaPress vary in price depending on their size; a variety of existing books of Wikipedia content that have been collected may also be browsed and ordered on the PediaPress site.
Feedback and problems can be reported on Meta; there is also a bug tracking system for the PediaPress code here.