The Traitor, a Detective and Spy Story by Somerset Maugham

In "The Traitor" by W. Somerset Maugham, the protagonist, Ashenden, is a British intelligence officer during World War I who is tasked with investigating an alleged traitor named Caypor. Caypor is an Englishman living in neutral Switzerland with his German wife and is suspected of passing information to the enemy. Ashenden befriends Caypor under a false identity.... I won't reveal more here....