Unneutral Murder (an Amos Lee Mappin mystery)

Criminologist Lee Mappin and assistant travel to Portugal during WWII to distribute a new secret code for the Allies and to help on the Underground Railroad smuggling freedom-loving Germans out of Nazi Germany. But the Gestapo has spies everywhere, and investigation of a murder close to home is hindered by the Portuguese authorities' need to remain neutral parties. With new friends, Mappin builds a case against the murderers while helping enemies of der Fuehrer secure passage to liberty. Unneutral Murder was published in 1944.