From the Wikipedia:
"Book swapping or book exchange is the practice of a swap of books between one person and another. Practiced among book groups, friends and colleagues at work, it provides an inexpensive way for people to exchange books, find out about new books and obtain a new book to read without having to pay. Because swaps occur between individuals, without central distribution or warehousing, and without the copyright owner making a profit, the practice has been compared to peer to peer systems such as BitTorrent -- except that hard-copy original analog objects are exchanged."
They currently offer nearly 700,000 books, list just 9 books with them that you are willing to trade away, and you immediately receive 3 free credits. Once credit equals one book.
- 2. BookMooch.com
BookMooch awards users with 1/10 of a point for every book they list and a full point every time users ship a requested book. Points are then redeemed one-to-one for books.
Typically the value for a paperback is 1 credit and hardcover and audio books are 2 credits. However, (paying) premium members have more flexibility in setting their buy & sell credit values.
TitleTrader also allows earning and trading points with DVDs, VHS, CDs, video games and magazines.
When you post something to trade away they immediately give you that book's 'market value' in Trade Credits.
in place of virtual currency (i.e. points) they use actual currency.
ZunaFish is the top site that uses direct peer-to-peer trading. The previous nine convert your books to a tradable currency, but ZunaFish aims to help you trade directly with someone else.
bookcrossing is defined as the practice of leaving a book in a public place to be picked up and read by others, who then do likewise.
There's an extensive summary of book swapping sites at http://web.archive.org/web/20070521121829/http://moochbooks.com/books/review/
It analyzes the trading models for each of the services listed above.