You can find lots of things at Library Genesis. http://gen.lib.rus.ec/
-kunkel321
Yes, but it is a Russian site. So, be careful.
-Arizona Hot
Are Russian sites dangerous?
-Merrikor
Not this one, no. This one is basically Pirate Bay for books and is highly regarded by users.
More problematic are those with pirated games and software.
-panzer
I've used libgen for years and have always trusted it (after my first few times there, of course). Used it to get thousands of digitised versions of books (that I own) and some that were simply unavailable anywhere even in print format. Also used it to get the translated version of some books too. Very useful.