A couple of the best resources I know of are actually on Reddit, in sub-reddits dedicated to cataloging freely available textbooks in various fields:<p><a href="http://www.reddit.com/r/csbooks" rel="nofollow">http://www.reddit.com/r/csbooks</a><p><a href="http://www.reddit.com/r/mathbooks" rel="nofollow">http://www.reddit.com/r/mathbooks</a><p><a href="http://www.reddit.com/r/eebooks" rel="nofollow">http://www.reddit.com/r/eebooks</a><p><a href="http://www.reddit.com/r/physicsbooks" rel="nofollow">http://www.reddit.com/r/physicsbooks</a><p><a href="http://www.reddit.com/r/econbooks" rel="nofollow">http://www.reddit.com/r/econbooks</a><p>And, of course, there is this classic site:<p><a href="http://people.math.gatech.edu/~cain/textbooks/onlinebooks.html" rel="nofollow">http://people.math.gatech.edu/~cain/textbooks/onlinebooks.ht...</a>