Apparently, one registers the book and then leaves it when you finish reading it. If someone picks it up they then go to the site and note where they found it. It seems to have hooked at least a few people:
Thus far, more than 11,000 people have registered more than 28,000 books they have “released into the wild,” as Hornbaker calls it. From his office in Kansas City, Mo., BookCrossing had spread to 36 countries.
Interesting, perhaps I will try it myself . . .