Flashback by Nevada Barr is one in a series of mystery novels featuring Anna Pigeon, intrepid US National Park ranger. She’s actual law enforcement, carries a gun sometimes, and is big into the derring-do.
This is book N of the series, but it holds up nicely if you have not read any others (as I had not).
It’s an interesting concept book, in which there are parallel mysteries of present-day and a series of letters from 150+ years ago, written by a family member staying at this same fort.
Give Nevada a star for this!