Of course, Libgen/SciHub doesn't have everything either, but it has more than any single legal resource I know of.
Also, I'd like to give a friendly reminder that anyone can upload books or scientific articles here. If it's books, so I'd encourage you to do that if you find resources elsewhere. ;)
If you have any questions, there's a forum for libgen at https://genofond.org/. Most of it is in Russian, but the staff are bilingual between Russian and English.