@Avidspark check out Anna's Archive as well. They have links to a lot of other open libraries and SciDB.
I found a ton of journal papers and even textbooks there. Very handy.
@LiberalLibrarian @Avidspark That one too. I think Anna's scrapes from them and zLib.
@kismatt @Avidspark
Also Libgen.is. That's my go-to.