Wikibooks is a Wikimedia Foundation project that aims to build a collection of free e-book resources, including textbooks, language courses, manuals, and annotated public domain books. It aims to help both (self-)instruction of students, and teachers in high-schools and universities.
As of January 2008, the largest Wikibooks is in the English language, with over 27,000 modules from over 3,000 developing books. The German and Portuguese follow, each has over 5,000 modules. The project started in July 2003 and now has over 84,000 modules from over 5,000 developing books, across over 50 languages.
Source : WikimediaFoundation.org