Hiking books are under the 796.51 call number and letterboxing is 796.14.
If you plan on putting the box on the shelf, get permission. Otherwise, the
box would more than likely be removed when a library worker finds it. We
currently have a letterbook in our stacks, which is a hollowed out book that
is made to look like all of the other books. Like someone else said, there
are tons of places to hide a box in a library without workers knowing, but
when thinking of box permanancy, the box would more than likely stay if the
library knew about it. Most librarians like the quest element of boxing and
typically welcome the additions. Many have the box behind the circulation
desk and seekers simply tell them a password or something along those lines.
Some have also gone with virtual letterboxes/books. Tom Cooch has a great
one that is both physical and virtual in Vermont.
Letterbooks are great diversions during the messier winter months when
getting outdoors becomes problematic.

-'boxing librarian