http://danaernst.com/resources/free-and-open-source-textbooks/
Here is a partial list of free and/or open-source textbooks. If you find one of these more helpful than another, please let me know. Also, take a peek at Rob Beezer's selection on this page. Moreover, the American Institute of Mathematics maintains a list of approved open-source textbooks. Also, check out the free computer science books by Allen B. Downey (Olin College) at Green Tea Press. Downey's Textbook manifesto is definitely worth reading.
