This seems like an interesting concept. It’s a site set up by students looking at bringing together in one location freely available online textbooks. The idea of introducing and using such material in lecture courses by both academics and the student population is rather an appealing concept in the present financial climate. This also works well for the idea of informal learning and open educational resources (OER).
The categories across the top of the page seem a little sparse at present, hopefully this will change soon. However, passing your mouse over the Books link reveals two menu items By Subject and By License. The By Subject item shows a useful list of subject to choose from.
If you have knowledge of any books that could be added to the lists, then there is an option to submit them.
Link: Textbook Revolution