Since nearly everyone on this group is a hiker or a "visitor to
National Parks" I had to recommend a Mystery Series by Nevada Barr.
All of her books are set in National Parks. The latest, Blood Lure,
is in the Waterton/Glacier National Peace Park straddling the border
of Montana and Canada. (and concerns Bears! recently a hot topic
here) One of my favorites is Deep South, set in the Natchez Trace
Parkway. Her descriptions of the heat and humidity of the south are
dead on (no pun intended). The protagonist is a park ranger. So far,
they have never come upon any letterboxes in any of her books!!

--Amanda from Seattle