The Secret Adversary
In addition to Poirot and Marple, Christie also created amateur detectives Thomas Beresford and his wife, Prudence "Tuppence" "née" Cowley, who appear in four novels and one collection of short stories published between 1922 and 1974. Unlike her other sleuths, the Beresfords were only in their early twenties when introduced in "The Secret Adversary", and were allowed to age alongside their creator. She treated their stories with a lighter touch, giving them a "dash and verve" which was not universally admired by critics. Their last adventure, "Postern of Fate", was Christie's last novel.