The A.B.C. Murders

Hercule Poirot receives a taunting letter announcing a crime to be committed in a town beginning with the letter A. Soon further letters follow, each pointing to another place and another initial, and the police face what appears to be a systematic killer working through the alphabet. Poirot, Hastings and Inspector Japp race to find the pattern and the person behind it. The book is admired as an early example of the serial killer story, and for the way it uses a seemingly senseless design to test the detective's understanding of motive.