You register a book on the website, put a label on it with a unique ID and the website address, then you leave the book somewhere for someone to find.
That's right, you give your book away to a random stranger.
Hopefully, someone will pick up the book, read it, then come to the website to let you know that they found your book.
Cool or what?
It's been around for a while, but I only just found out about it via i-am-bored.com.
So everyone should join up and start sending your books out into the wild.