More Open Textbook Resources
There are other sites to get free (but perhaps not as "open") books and titles to use in courses.
The publisher O'Reilly has offered a number of Open Books - books with various forms of "open" copyright - over the years. The reasons for "opening" copyright, as well as the specific license agreements under which they are opened, vary.
Perhaps a book was outdated enough to be put out of print, yet some people still needed the information it covered or it has historical significance. Or the author or subject of a book felt strongly that it should be published under a particular open copyright. Maybe the book was written collectively by a particular community, as in the case of their Community Press books.
But there's more to making Open Books available online than simply adopting an open license or giving up rights granted under copyright law. The print books need to be converted to a digital format so that they're accessible via the web. O'Reilly partnered with two innovative nonprofits, Creative Commons and the Internet Archive, to solve the licensing and digitizing challenges involved in bringing Open Books to readers.
For example, here are some titles they offer that are about the open movement itself.