The Secret Adversary (Tommy and Tuppence Series)

First published in 1922, The Secret Adversary is Agatha Christie's second novel. In it, she introduces Prudence "Tuppence" Cowley and Tommy Beresford who return in several other Christie books. Tommy and Tuppence decide launch a detective firm together and their first case involves finding a missing person. They soon find themselves caught up in a chain of events that began with the sinking of the Lusitania.