A Miss Marple Nightcap: The Blood-Stained Pavement by Agatha Christie

Another day, another episode wherein Miss Marple outsmarts all those foolish enough to underestimate her. This one is quite a pitch-black little story, as evidenced by its rather fantastic alternate title in the U.S.: Drip! Drip! And perhaps also by the fact that it's set in a place called "Rathole."