A large number of open educational resource textbooks have been made available. Many are peer-reviewed! In addition, many OER sites provide related, non-textbook, course content that is also open and available for anyone to use (and remake, if they would like). All of this content is available freely on the open web for faculty and students.
Here's a list of sites that include freely available OER textbooks and other course/teaching materials (in no particular order). Some are discipline-specific, and some contain multidisciplinary resources.
Brief Descriptions of each - Coming Soon!
Needs some ideas for implementing electronic textbooks in your courses? These organizations have useful research and best practices to check out.