N or M? by Agatha Christie

Published in 1941, N or M? features detecting husband-and-wife team Tommy and Tuppence Beresford who are too old to participate in the war but long to contribute in some meaningful way. Tommy is approached by British Intelligence to go undercover to a seaside boardinghouse, which is suspected of harboring Fifth Columnists, English people who sympathized with Germany and committed traitorous acts on its behalf. Even though Tuppence is supposed to be in the dark about the assignment, she overhears the plan and shows up at the boardinghouse as a paying guest. Agatha Christie wrote this mystery while living in London, experiencing blackouts, rationing and air raids. She also returned to the work that she had done in World War I, acting as a dispenser in a hospital. N or M? is a typical Agatha Christie mystery, easy to read and hard to put down.