Death of a Saboteur (an Amos Lee Mappin mystery)

Crime author and occasional detective Amos Lee Mappin has entered the war effort as an undercover investigator, trying to infiltrate a spy ring. A shooting doesn't help matters, and Mappin must scramble to maintain his cover while keeping the police in the dark about his mission. A dangerous new foe enters the fray and friends end up in peril as Mappin is forced to play out the game of cat-and-mouse to an exciting finish. Death of a Saboteur was published in 1943.