Has anyone joined
BookCrossing.com and used its services (which are free)?
For anyone who hasn't heard of it the basic idea is that you 'release' books into the wild at certain places (Crossing Zones). These could be cafes, park benches, or even trains. There is a sticker inside each book asking the finder to log it at the Book Crossing web-site. It's basically a book-swapping service, but it also allows people to see where they can find books, or track what has happened to books they have released. There is an online community feel to the site, rather like BBFans.
I thought I'd give it a plug anyway.