What do you do wth books you no longer wish to own? Recycle them? Donate them to Goodwill? Try to sell them at the used bookstore?
With Bookcrossing there is another option: tag your books to track their whereabouts, then release them into your nearby area. With a bit of luck, the book will be found soon, and a journal entry will be recorded on the Bookcrossing site as to its current whereabouts.