Don't toss your books in the trash or just dump them off at some thrift store...
Here are some interesting ways to swap books for free:
(free but the page looks complicated)
"The practice of leaving a book in a public place to be picked up and read by others, who then do likewise."
"Give books away, Get books you want."
"Trade books using a simple point system."
In my opinion BookCrossing is the coolest. People literally leave a book "out in the wild" laying around for anyone to pick up. This is gonna save me a few trips to the dump/shooting range/thrift store.
(yes I said shooting range)