Benn, James R., Blue Madonna: A Billy Boyle WWII Mystery
It's late May 1944. Captain Billy Boyle is court-martialed on spurious charges of black market dealings. The chief witness against him is Archie Chapman, the London gangster last seen in Rag and Bone, and Boyle is convicted. Stripped of his officer's rank, reduced to private, and sentenced to three month's hard labor, Boyle is given an opportunity: he can avoid his punishment if he accepts a dangerous mission. In the town of Chaumont, a "petite chateau" on the outskirts of town serves as a safe house for the Allies. The chateau has a secret chamber and tunnel, first used to hide escaping Huguenots in the 17th century. But someone is killing the people who have passed through the safe house. Two officers, one a British escaped POW and the other an American pilot, have been murdered. Billy is parachuted in as part of a three-man team on June 5, 1944, the night before the Normandy invasion. Billy must solve the mystery of who is behind the murders, then lead a group escape from France back to England, with both the Germans and a killer hot on their heels.