The Letterboxer's Companion, 2nd Edition

Letterboxing is an intriguing mix of hiking, puzzle solving, treasure hunting, and rubber stamp artistry, topped off with the thrill of discovery. The basic idea is simple. Clues, usually posted on a web site, will lead the person who correctly follows them to the secret location of a letterbox. The letterbox contains a unique rubber stamp and a logbook. The searcher carries a personal rubber stamp and logbook. When the letterbox is found, its finder stamps the box's logbook with his or her personal stamp, and stamps the personal logbook with the stamp in the letterbox. The letterbox is then secreted away again.