PaperBackSwap.com lets you do just that:
instead of buying a book, you can get it free by swapping it. It’s basically a great big library—only you can browse from your chair, and your chosen book will be delivered to your door. Think of it also as a way to de-clutter your bookshelves…until you fill them u p a g a i n w i t h your new reads. Registration is free,and you only need a valid email address and postal address to get started—plus a list of all the books you want to place on the site for other members to “buy”. As soon as you’ve listed your own books, you can start browsing over a million on the site (search by title, author or subject), and best of all, shipping is free. You only pay to send your books to the people who have requested them; anyone sending you books will pay the shipping fee. So the most you will ever pay is the cost of postage to send a book—forget about sky-high “new release” prices. You don’t even need to buy an envelope: you simply wrap the book you’re sending in two pieces of printer paper, on which you’ve already printed the receiver’s address.
Get swapping!