Openstax: The Bill and Melissa Gates Foundation and other have funded this open textbook project.
College Open Textbooks: a collection of twenty-nine educational non-profit and for-profit organizations, affiliated with more than 200 colleges.
Digital book index: links to more then 150,000+ digital books from various universities and publishers. Around 100,000+ of these books are available for free.
Project Gutenberg: the oldest digital library in the world offers more than 43,000 e-books, completely gratis.
University of Pennsylvania: a mini-site which links to free textbooks available on the internet. It has over 1 million free eBooks.
Boundless:They use public sources to compile information and align it after the big selling college textbooks.
Flat world knowledge: offers free and open source college textbooks. These textbooks can be read online using their easy to use online reader.
Will We Soon Be Able to Resell E-Books and Digital Music?