The Giver by Lois Lowry
The Giver is another one of those books that suffers from "Wait, it's over? WHAT THE HECK!?" syndrome. It's a brilliant, well-written novel with a lot to say that just seems to suddenly end with no clarity whatsoever. Apparently, things are explained a bit in the fact that it turns out to be the first novel in a quartet, although you wouldn't know that by looking at the cover. It doesn't help that it took seven years for the next book in the series to come out!