The Secret Adversary

In 1915, as the Lusitania sinks, a man hands a packet of secret papers to a young American woman. Four years later, in post-war London, two old friends, the cheerful, penniless Tommy Beresford and the bold Prudence 'Tuppence' Cowley, set up as 'Young Adventurers', willing to go anywhere and do anything, and are soon caught up in the hunt for the missing papers and the girl who had them, pursued by a criminal mastermind known only as Mr Brown.

Published in 1922, this was Agatha Christie's second novel and the first featuring Tommy and Tuppence, who would appear in her books until 1973. It is a fast, light-hearted thriller rather than a puzzle. It pairs well with The Mysterious Affair at Styles.