e-books
Kindle Popular Classics "hundreds of thousands" of free or cheap e-books from Amazon
HathiTrust Over 3 million works available as full-text e-books
Internet Archive over 15 million free e-books to read and download
Open Library over 2.3 million e-books
Project Gutenberg over 57,000 free e-books
Directory of Open Access Books (DOAB) a community-driven discovery service that indexes and provides access to scholarly, peer-reviewed open access books
National Academies Press The publications of the National Academies of Sciences, Engineering, and Medicine cover a variety of topics and are available free online.
audiobooks
LibriVox free, public domain audiobooks