"Library Genesis or LibGen is a search engine for articles and books on various topics, which allows free access to content that is otherwise paywalled or not digitized elsewhere. Among others it carries PDF's of content from Elsevier's ScienceDirect web-portal."
"As of November 2017, the Library Genesis database contains more than 2.1 million books."
That said, as of this moment, libgen.pw is up, even if libgen.io isn't.
This is also a good time to remind people that there is a torrent backup. If you have a seedbox or just space on your computer consider downloading a few and seeding!
Having gone through Library Genesis' Wikipedia entry a couple of times, I was surprised at myself for not having thought about this before!!