Project Gutenberg - A library of over 60,000 free eBooks, with focus on older works for which U.S. copyright has expired.
Libretexts project, a leading, non-commercial open textbook organization initiated at the University of California, Davis.
Open Textbooks Library - library currently includes 766 textbooks, with more being added all the time
JSTOR E-Books - Thousands of Open Access ebooks are available from top scholarly publishers, including Brill, Cornell University Press, De Gruyter, and University of California Press
Open Book Publishers - Independent open access academic publisher of monographs. Free online editions of every title in PDF, HTML and XML formats that can be read via our website, downloaded, reused or embedded anywhere.