Woah! You're confusing a couple of things here SG. Masquerade was the first of a genre now known as 'armchair treasure hunts'. There have been many similar books/hunts published since, and a club called the Armchair Treasure Hunt Club exists. Bamber Gascoigne wrote a book 'Quest For The Golden Hare' about the hunt and its solution.
Letterboxing is a phenomenon associated with Dartmoor but which has spread, minimally, to other areas of the UK/world. Thousands of little boxes are hidden all over the moor, each containing a rubber stamp and visitor's book. Letterboxers find them, either by chance or from clues, and collect impressions of the stamps on cards or in books. I have well over 2,000 such stamps, many of which are beautifully designed and cover a wide range of subjects. A fascinating hobby which takes you to places you may otherwise never have visited.