epubBooks.com: free public domain eBooks that you can download in the EPUB format.
Google Books: Previews and full-text ebooks. Full-text books are out of copyright, or books in which Google received permission from the publisher to reproduce.
Internet Archive: Ebooks and Texts Archive: contains a wide range of fiction, popular books, children's books, historical texts, and academic books. Internet Archive declared National Emergency Library; this means multiple users can check out eBooks at the same time for 13-days. Users only need to create an Open Library account.
Project Gutenberg: first and largest single collection of free ebooks that are out of copyright in the United States.
OAPEN Library: Open Access Publishing in European Networks (OAPEN) Library contains freely accessible academic books, mainly in the area of humanities and social sciences.
BioMed Central: part of Springer/Nature, BMC publishes open access journals in science and medicine.
Open Book Publishers: freely-available ebooks and e-textbooks in the Humanities and Social Sciences.