http://www.quora.com/What-are-some-alternatives-to-library-nu

Scribd, Oyster, and Kindle Unlimited all give you a "netflix for books" type experience where you pay a monthly fee of about $10 and read as many books as you want (not newer books, unfortunately). (Kindle Unlimited might be better if you have a non-Kindle-Fire kindle device it will work well with, but since publishers don't like Amazon it will never have as good quality of a selection as the other two.) Your local library may also have ebook lending options.

BTW, if you want papers rather than books, this thread (actually, use this more recent one) may be of interest, esp. this site or this site or http://reddit.com/r/scholar or the #icanhaspdf twitter hashtag

Note when using libgen search engines, gwern writes: "I've noticed the Libgen search engines seem to have problems with long titles and/or colons" so you may wish to strip those.

Another cool site. reddit discussion of book piracy. List of LibGen mirrors. EBook search engine? Another list of sites. And another even longer one. Quora thread compiling sites. Another list.