Flashback - Nevada Barr

Flashback is an Anna Pigeon novel.  It's the first one I've read but not the last.  Anna is a park ranger and she's got temporary duty at the far edge of the Florida Keys in Dry Tortugas National Park.  She's temporarily in charge because her predecessor seemed to go insane.  Now she finds herself slipping into a mental fog too.  Finding the cause is just one of her problems.  It turns out that everyone working for her is not what they seem.  There's a little romantic angle to the story but I like that it doesn't interfere with the mystery and the solution.  Highly recommended.