The Open Textbook Library utilizes a network of many foundations and universities to pull together textbooks written by faculty and reviewed by faculty.
openstax is housed at Rice University and is supported by many foundations. The textbooks found in openstax are also listed in Open Textbook Library.
Open textbooks from British Columbia.
From the State University of New York system (SUNY), these are textbooks written by their faculty. I suggest browsing them by subject.
MIT Open Courseware - Open Textbooks Page
From the University of California and California State University systems in collaboration with several other institutions.
The DOAB emanates from The Hague and they have a strong vetting process for allowing open books in their repository. They are a member of the OASPA (Open Access Scholarly Publishers Association) and are international in scope.
Hathi Trust is a partnership between academic and research institutions and offers millions of titles digitized from libraries around the world. Books and journals were scanned by Google or individual institutions. Look at the record for each item for publishing rights. Most will be public domain.
