It’s a swapping scheme, whereby you list books to share, and in return choose books to read. “At PaperBackSwap.com, members list at least nine paperbacks, earning three credits. Credits allow them to search available titles and choose up to three. Senders pay the postage. They receive one credit for each book they mail, enabling them to order other titles. The website formats a mailing wrapper. The sender then prints out the wrapper, adds stamps, and mails the book.”