Another Anna Pidgeon mystery that did the trick for a long drive. She's a very likeable character - middle aged abd slightly jaded. This park ranger really makes the rounds of the national park system.
In this one she's stationed at Fort Jefferson, a former prison in the Dry Torguas off the Florida Keys. It's a parallel story about human smuggling and Samuel Mudd, who was imprisoned at the fort for his participation in the Lincoln assassination.
Another great locale and interesting information about the fort.