I'll make it THREE cheers for library letterboxes! This idea occurred to me at one point a couple years ago, but I never did anything with it. Kudos to all those who did, recently and in the past.
When I was a kid, I hollowed out one of my mother's favorite books -- without permission, of course -- and used it as a camouflaged cache for small private items. I'm sure I got the idea from a movie or TV show. Letterboxing in general tugs at the same strings that made that delicious fun (until my mother found out...), and so for me letterBOOKing pulls a very central string!
Besides, librarys are already places of discovery. What better place for a treasure hunt?!
About clues:
Obviously there's PLENTY of room in the world for all kinds of cluesets, but I'd like to encourage some of the next few people who put out letterbooks to create cluesets of varying degrees of difficulty. Riddles, in particular, lend themselves to LOTS of ambiguity and I'm concerned that they can be impediments to beginners -- enthusiasm dampeners, if that makes sense.
I definitely believe in providing challenges for a wide variety of skill-levels, "learner" types, and so on. Let's set the bar high in some of our cluesets to challenge the best and brightest! But let's make sure the young, the beginners, and the differently abled can all play the game.
-Mark (never good at story problems)