In the Shadow of Monsters by GS Carberry

1940: The French army is collapsing before the Nazi onslaught. Britain will surely be next. Niki Frolov, a refugee soldier with a terrible past, is recruited by a secretive new British intelligence agency determined to keep the fight alive. Isolated behind enemy lines and pursued day and night, he will place his heart in the hands of a most unusual young woman and his life in those of an enigmatic stranger, a man with a cause worth fighting for.

In a country torn to pieces by war and poisoned by collaboration and betrayal, they must turn a fractious group of eccentrics, partisans and political enemies into a united resistance just to keep the dream of a free France alive.

But they are in a race against time, pursued by a monster who hunts them day and night: Klaus Barbie, the infamous Butcher of Lyon.