A Postcard from Hamburg: A WW2 spy story

WW2, 1943. The third thriller in the Black Orchestra series. Kurt Müller, fugitive member of the German resistance, is working for British Intelligence in London, when he learns that Gudrun, the love of his life, has been abducted from their home in Ireland by the Nazis. He goes AWOL to find her, making his way to Hamburg across Occupied France with his friend Erika, an SOE operative on a secret mission.