Halfchan keeps promoting the IRC model of book sharing. I'm not so sure this is in proper keeping with modern image-board culture. Based on my limited expeditions into IRCland, if you can't find something elsewhere it won't be found on #bookz either. Plus, there's a lotta extra effort, expense, and learning curve you have to put in to guarantee anonymity and privacy over IRC. At least it's something to be aware of for the more curious to try.
The Libgen blob became the single greatest library on Earth. This should probably be your first stop for searching.
/pdfs/ here on 8chan is an option, albeit they're best fit for the cultural warrior types. Worth a quick look though.
The onion lands (or the even more exotic network frameworks being developed) host a growing field of repositories, but I personally hate seeing any Tor exclusive resources discussed on a clearnet service. The September that never ended has ended. No more hand-holding please! Leave the normalfags to stew in their facebook/twitter/google enclaves so we might enjoy what remains free and worthwhile for a while longer.
