Project Gutenberg offers over 59,000 free eBooks. Choose among free epub and Kindle eBooks, download them or read them online. You will find the world's great literature here, with focus on older works for which U.S. copyright has expired. Thousands of volunteers digitized and diligently proofread the eBooks, for enjoyment and education.
Open Culture Free eBooks. http://www.openculture.com/free_ebooks
InTechOpen https://www.intechopen.com/books
is a repository of academic books in the areas of physical sciences, engineering and technology, life science, health sciences and social sciences and humanities.
The MERLOT http://www.merlot.org 
collection has over 5,500 open textbooks. Click on a link to your subject area and you will get a list of results for open textbooks in the subject and its sub-disciplines. The MERLOT indexing tools enable easy browsing and searching throughout the entire collection of open textbooks.
https://openstax.org OpenStax is a nonprofit educational project based at Rice University. The mission is to supply tools so that every student is successful in the classroom.
