You post a list of the books hanging around your house and look over the lists of other members. If someone wants one of yours, you receive an email with the address. As the sender, you pay the postage, currently $2.38 for up a pound, but you earn credits with the swap service, which then allows you to receive books.
When you request books, you use your credits and receive your reading for no money at all. For titles not currently available, there's a wish list — you'll be notified when someone has a copy to trade.
Joan Rattner Heilman writes about good deals and how to find them.