If you want to trade titles in your personal library, or if you missed a a title and you want to acquire it, try BookMooch : a community for exchanging used books.
BookMooch was launched last week, and already, it has around 4,000 members “mooching” used books. To start mooching (exchanging) used books, one has to register and start earning points. The number of points you earned determines your eligibility to particular book that you want to receive. You can earn points by posting books that you want to give away. To facilitate the exchange of books, members will shoulder the shipping costs. Giving away and receiving books have their corresponding points that is credited to your account.
Basically, BookMooch is similar to Library Thing, though it goes beyond by simply cataloging personal collection to include exchanges as well.