OpenStax is a non-profit based at Rice University. All textbooks are downloadable as free PDFs, for Kindle, or for iBook, or read online, and have been peer-reviewed. Paper copies can be purchased for $38.50-$52.00. Strong in math, science, economics, and one book each for U.S. history, sociology, psychology, and American government. Test banks are available with sign-in. OpenStax CNX is a closely-aligned platform for other faculty to mount and share textbooks for global consumption, which includes a textbook editing tool.
All textbooks are downloadable as free PDFs, epub, or read online. Content is entirely different from OpenStax, including movies, logic, anthropology, and more.
That it's from MIT makes it no surprise that engineering figures huge in this collection, with a little bit of management/logistics. Downloadable options vary, but most have chapter-level PDF downloads, though some are only read online.
Downloadable collection of 22 textbooks from various disciplines, which are downloadable as e-pub, but not PDF. Primarily to be read online. Has many colorful illustrations accompanying the sampled texts.
A catalog of "free, peer-reviewed, and openly-licensed textbooks" out of the University of Minnesota. While you can submit books, this site attempts to index materials found in OpenStax, MIT, and others.
Cooperative project providing free textbooks as well as linking to materials from OpenStax and others.
A commercial site that allows faculty to look at a text, use it, modify it, and adopt it. Viewing, downloading, and paper copies cost students in a range of $30-75. Faculty can also submit a text for publication through Flatworld.
Similar to Flatworld. Texts are adopted by the professor, then purchased by the students at tiered pricing based on functionality of use, from viewing to downloading, black and white printing to color printing bundles.
The cost of most textbooks runs from $40-80. Texts can be rented for a variety of days, depending on the book, and provides online access. Faculty may review a book upon request.