Better World Books is one of my favorite book-sellers of used books. They give a big portion of their profit to support worldwide literacy causes (there is little more important for world peace then to get people to read).
And they also have a great Rapleaf reputation.
I'm not sure if I am allowed to tell anyone about it, but they're doing a friends and family discount code of 10% off for the rest of 2006. All you have to do is enter BETTERWORLD_FRIENDS at checkout. And they also have free shipping in the US, $2.97 worldwide. Pretty cool -- check them out.