Bookcrossers discussion

So, I read Book Scavenger in which the kids play a wildly successful game that is a mix of bookcrossing, geocaching, and letterboxing. All I could think about was 'man o man I wish ppl were that interested in logging their finds and otherwise participating in BC!' Cute book though.

So, I also do a bit of geocaching, and letterboxing looks interesting so I might get a stamp so I can do that....

Do any of you do anything other than bookcrossing?
Do any of you know of any other books on any of these topics?