Here a list of Website where you can find very nice books (in pdf/djvu/chm format)
http://it-ebooks.info/
http://www.allitebooks.com/
http://libgen.io
http://bookzz.org/ or http://en.bookfi.org/
http://mirknig.com (in Russian, but you can use google translator on it)
http://booksee.org/
http://bib.tiera.ru
http://ebook-dl.com/
http://lib.org.by/
http://mirtitles.org/ specialized in the MIR titles Good old stuff!
