This has probably been posted before but its a fantastic idea.
You register books with them, which are then given an ID number, then "release" them. Idea is that someone finds them, logs onto the website and registers what they thought of it, then passes the book on again. Basically you can track wherever the book goes, assuming all finders register its movement, and find out what they thought of it.
You can also hunt for books that other people have "released".
I've released one in my city today, and plan on doing more and seeing what happens.
And before this gets moved to bookworld, it's already in there. So