A few months ago, I came across this note that opened up a whole new world of free books. It was truly a blessing. But for the past month or so, libgen.org's and its mirrors' servers have stopped responding. I have checked out these mirrors and they're not working:
libgen.net. This site actually redirected me to another website that's called Libgen project. I tried downloading a book from there, but it was "damaged beyond repair", according to Adobe reader. Plus this site had a lot of ads.
libgen.education. This is also the source of material for the mirror gen.lib.rus.ec, which appears to be running, but alas, it only contains the links.
bookfi.org. This site also has a lot of ads and I had problems when I tried to download a book.
If any of you have discovered a functioning mirror, please let me know.