The Alphabet Murders book by Agatha Christie

A series of horrific murders have been committed. In deciding who will be the next victim, the criminal turns to… the alphabet. The unsurpassed detective Hercule Poirot enters the game. Before each murder, the unknown person sends the detective a letter in which he writes where and when blood will be shed again, but the police and Poirot each time fail to save the victim. The chain of evidence leads to a potential killer - an inconspicuous traveling salesman, stocking seller Alexander Cast. But suddenly the alphabet scheme fails, and Poirot realizes that the killer is someone else…