Basically it's a huge way of recycling books. You can request books from others and send your used books to people who'd love to read them. The cost? Whatever it costs for you to ship your book to someone's house (usually around $2.) For every book you share, you get a credit to order a new book.
I posted a bunch of books and had requests for 6 of them within 30 minutes. So now I'll get 6 new books!
By the way ... does anyone know where I can find a new LiveJournal Mood Theme? I'm growing a smidge tired of my Disney characters.