Anna's Archive — 61M+ books and 95M+ research papers for free.
https://annas-archive.org/
https://blossom.primal.net/0988948b9c8aab1df59927708a87e158ee31aa968090aaa833d33b3f77316baa.png
Interesting, that URL gets filtered by CloudFlare's "adult/malware blocking" DNS.