In order to quickly get the desired results, I gave him a few rare and useful search tips. That got him first to visit http://www.freebookspot.me/.
I don't whether he was able to find what he wanted but I got myself a few game developing books that I have been looking for but did not put in serious effort to search them.
http://www.freebookspot.me/ has got thousnads of free e-books on scientific, engineering, programming, fiction and many other books. You don't need to create an account to download free e-books.