Till Death Do Us Part by John Dickson Carr
John Dickson Carr is considered a master of the locked room mystery and Till Death Do Us Part is a good example of that. This is the first Gideon Fell book I think I've read, but it worked fine as a standalone.
Dick Markham with his fiancee, Lesley Grant, arrive late to the village fair. Events lead to Lesley accidentally shooting the fortune teller, renowned criminologist Sir Harvey Gilman. Later Markham hears from Gilman the story of Lesley's life as a serial poisoner. Soon Superintendent Hadley and Dr. Gideon Fell become involved when a murder occurs in the village.
It's an enjoyable mystery. The characters are the usual odd lot that lives in fictional small towns, with assorted secrets and jealousies. There is plenty of misdirection, lots of red herrings, and several people who aren't who we think. I did not guess who the killer was or how they managed to pull it off. It's a quick read, but I think...