Aurore by Graham Hurley

Aurore connects Billy Angel, an RAF wireless operator selected for a covert mission to occupied France, with Helene Lafosse, a French woman keeping unusual company in her small family chateau in the depths of the Touraine.

Helene has begun an affair with senior Abwehr Intelligence officer and in return he has turned a blind eye to the succession of Jews, refugees, resistance fighters and downed Allied airmen to whom she offers shelter. MI6 want to exploit their relationship and plant a false lead about the D-Day landings. It falls to Billy Angel to find his way to the Chateau de Neune where he must win Helene's confidence and share the tiny piece of the intelligence jigsaw that will only make sense to her German lover.