MIT Open Courseware was first created in 2001 and is praised for it’s well-made and accessible courses. However, that is the main focus: courses that a person can walk though to teach themselves subjects taught at MIT and beyond. Some of the material from these courses are free, but not all of them have textbooks and not all are open.
While hunting through the site, I specifically looked for courses with online textbooks, which is the most narrowing that one can do when looking for books. I clicked through some courses and the books looked substantial, but the best ones have a separate menu heading for “Textbook.” Those without tend to be smaller, but often a decent size for the material. Some have integrated Java applets, which can not be downloaded simply into a pdf.
There about 20 actual open textbooks on the site, and they are not spread across the subjects evenly, with most of them in the engineering section. The resource is decent, but not good for a widespread approach to using open textbooks.