Open Access ebooks
The OAPEN Foundation is a not-for-profit organisation based in the Netherlands, which is dedicated to OA, peer-reviewed books. It provides the OAPEN Library, a central repository for hosting and disseminating OA books, and the Directory of Open Access Books, a discovery service for OA ebooks.
Other sources of free and OA ebooks include Hathi Trust, NCBI Bookshelf, The National Academies Press, Project Gutenberg, Bartleby.com, Internet Archive eBooks and Texts, Read Print, Open Humanities Press, and O'Reilly.
You can search for individual OA ebook titles on StarPlus.