Some neighborhoods have small boxes set up for people to put in a book and then they can take a book (or not). Nice idea if you can find one nearby. Personally, we don't have a lot of room for books anymore, so we only keep ones that are really dear to us. If I buy a book (sometimes one just has to, you know), I read it and donate it to our library. They sell them to the public and that helps boost library funding. Win-win.