For the literature about dumpster diving see here.
Books can often be found while walking through affluent Western cities. Just keep your eyes out for random boxes in the street. guaka has found books in Paris, Amsterdam, Cologne and several other cities.
Believe it or not, bookstores will throw away perfectly good books. Sometimes they rip off the cover, sometimes they don't. Try behind Half Price Books in the US.
Fun tip: if you don't want to keep the awesome books you found yourself, try giving them away to unsuspecting strangers.
You also can find free books, and share books you rescue through "Little Free Libraries" which are neat little decorated boxes in public areas, all over the world. Find those close to you at this link