Books can be paperback or hardback and the only cost is the shipping to send your book out. The more books you give away the more books you can get.
It may not be the best service for people not willing to part with any of their books, but I thought it was great since I do occasionally find myself with a book I no longer want. Before finding Paperback Swap I would donate the book to my local library, now I can give them away to other readers in exchange for books I want to read and own. To open an account you must post 10 books you wish to give-away, this was difficult as I only had 9 books to post, but I figure I can always pick up an inexpensive paperback at a library sale in the next week and launch myself into free-book status.
At any rate, I thought this was such a great concept I wanted to pass the link to my readers.