The Secret Adversary (World Classics in Large Print, British Authors)

This was Agatha Christie's second novel. It introduces the characters of Tommy and Tuppence, who feature in three other novels and a collection of short stories. The young couple Tommy Beresford and Prudence "Tuppence" Crowley find themselves out of work and, offering their services to any takers, find themselves involved in the mystery of a passenger who survived the sinking of the Lusitania in 1915 with a draft treaty that could be dangerous in the wrong hands.