Flexbooks are free online textbooks that can be customized to meet your needs. One of the better websites that we like is http://www.ck12.org/.
This website has flexbooks available in a variety of formats: computer (online view), .pdf (computer and mobile view), Kindle, and iPad. With this kind of flexibility you can find or create a book for students on just about any subject.
Have you used flexbooks? Share your experience here!