Open Culture, has just published a collection of more than one hundred free textbooks available online. All of the books in the collection can be viewed for free and most can be downloaded for free. If you’re looking to replace the textbooks used in your classroom or you’re looking for supplemental texts, the Open Culture collection of free textbooks is a great place to start your search.
http://www.freetech4teachers.com/2010/08/100-free-textbooks-from-open-culture.html