Note: This list might contain some sites that have concerns over legality and copyright issues. In all such known cases, the corresponding Wikipedia page has been linked, instead of linking directly. Before going through this list, please read this thread on academia.stackexchange.com.
- HN Search 'books'
- Open Textbook Library
- Project Gutenberg
- Standard Ebooks
- The Internet Classics Archive - Bringing the wisdom of the classics to the Internet since 1994.
- MIT OpenCourseWare Index of Online Textbooks
- Library Genesis
- Global Grey Ebooks
- Colin Stebbins Gordon
- The Pirate Bay
- This r/learnmath thread.
- PeerJ - Peer-reviewed & Open Access journal publishing primary research and reviews in biology, life sciences, environmental sciences, and medicine.
- unpaywall.org - Unpaywall Firefox add-on
- Google Scholar