What to do with old books is a perennial library problem - and finding new material, for a good price, is another. PaperbackSwap.com* solves both problems at the same time.
As LinC-list member Virginia P explained in a recent post: "This is a book swap site where you ship a book to someone who wants it, and get a credit to order a book for yourself. You post books you want to swap online and create a wishlist of titles you want. You specify what you will accept condition-wise. Sometimes you might have to wait a while for something you want to show up on the website, but there are over 4 million items listed. "
To get started they even offer credit to get two free books just for posting 10 titles that you want to swap.
Alas, it's only available in the USA.
*The name is a bit of a misnomer; members can swap any kind of book as well as CDs and DVDs