WikiBooks is a trend-setting effort in promotion of free and open learning. It is a collection of free textbooks with supporting book-based texts, that is being written collaboratively. The project was started on July 10, 2003 and there are 5398 modules that are being worked. WikiBooks can be a good source of learning in various topics.
All of the site’s content is covered by the GNU Free Documentation License. Contributions remain the property of their creators, while the copyleft licensing ensures that the content will always remain freely distributable and reproducible.