Solemn Graves (Billy Boyle WWII Mystery Series #13) - James R. Benn

July, 1944, a full month after D-Day. Billy, Kaz, and Big Mike are assigned to investigate a double murder, close to the front lines in Normandy.

An American officer and a member of the French Resistance were found dead in a manor house outside the town of Trevieres. The investigation is shrouded in secrecy, due to the highly confidential nature of the American unit headquartered at the Manoir de Castilly; the 23rd Headquarters Special Troops.

This vague name covered a 1,000-man unit with a unique mission within the US Army: to impersonate other US Army units in order to deceive the enemy, causing them to think they were facing large formations, when in reality it was the 23rd (they called themselves The Ghost Army) creating deceptions by radio traffic, dummy inflatable vehicles, and sound effects. Not even the units adjacent to their positions were to know what they were doing.