"Many know Saylor Academy for these books, but our main efforts go into creating open online courses. We encourage educators to use our courses, in whole or in part, in their own classrooms. Visit our course site to find more textbooks and curated open educational resources (OER)."
SPC is one of the registered institutions. You can create an account and get access to free eTextbooks in their collection.
"A collaborative book authoring website, where users from all over the world work together to write textbooks and other types of instructional books on many topics."
"The Open Textbook Library provides a growing catalog of free, peer-reviewed, and openly-licensed textbooks."
"The B.C. Open Textbook Collection is home to a growing selection of open textbooks for a variety of subjects and specialties. Discover open textbooks that have been reviewed by faculty, meet our accessibility requirements, and/or include ancillary materials (quizzes, test banks, slides, videos, etc.)."
*All descriptions are taken from the corresponding organization's website.