While some books are original, others began as text copied over from other sources of free content textbooks found on the Internet. All of the site's content is covered by the GNU Free Documentation License. This means that, as with its sister project, Wikipedia, contributions remain copyrighted to their creators, while the copyleft licensing ensures that the content will always remain freely distributable and reproducible.
Wikibooks differs from Wikisource in that content on Wikibooks is expected to be significantly changed by participants. Raw source documents such as the original text of Shakespearean plays are hosted on Wikisource instead.
The project is working towards completion of several textbooks in numerous human languages, which founders hope will be followed by mainstream adoption and use of text developed and housed there.
Wikibooks has many incomplete texts, and even the comprehensive texts (books rated at the highest level) are of poor quality. The wiki model encourages the creation of abortive book projects that linger indefinitely without being improved or deleted, and it can be extremely hard for a visitor of the Wikibooks site to find any high-quality, completed books. Wikibooks also inherits all the criticisms levelled towards wiki-process editing in general.
The counter arguments to these criticisms are that Wikibooks content is available for almost continuous peer review, and thanks to the wiki-style editing, this project also inherits all of the strengths of wikis. For popular titles and items that gather interest from several participants, the quality of the content tends to improve rapidly. Noted books that have achieved this are all highly technical books, especially related to computers and the Internet, as well as linguistic books related to (human) language learning.