Since it launched in 2012, OpenStax has produced 20 textbooks in a number of subjects. As open educational resources, the textbooks are free to view, download, and modify. All content is sourced by publishing experts and vetted by peer reviewers, and textbooks can be viewed online, downloaded as a PDF, or purchased as an iBook or traditional print textbook. The focus has been mainly introductory textbooks, particularly math and the sciences.

Interested? Check them out at: http://cnx.org/