"The Mysterious Affair at Styles" (1920) is Agatha Christie's first novel, and introduces the characters of Hercule Poirot, Chief Inspector Japp and Captain Arthur Hastings:
In the midst of World War I, the residents of Styles wake one morning to find Emily Inglethorpe has been fatally poisoned. Captain Hastings, staying with the family, enlists the help of his old friend, Belgian detective Hercule Poirot. With the evidence mounting against one member of the family, Poirot uses his technique to prove who really killed Emily... ---
"The Secret Adversary" (1922) introduces the characters of Tommy and Tuppence: Set in 1919, the young couple form a partnership, hiring themselves out as "young adventurers". Their first case, however, is more of an adventure than they expect...