The Mysterious Affair at Styles
Agatha Christie's debut novels introducing the world to Hercule Poirot and Tommy and Tuppence. The publisher says, "These two books changed detective fiction forever and revolutionized the mystery genre."
The very first Hercule Poirot novel! When Emily Inglethorpe dies suspiciously after marrying a man 20 years her junior, her nephew, Captain Hastings, recruits his old friend Hercule Poirot to help in the investigation.