Manning Coles 
The Fifth Man [EPUB ebook] 

Colemore has escaped from prison to the Continent, only to be caught up in the chaos of World War II. He takes the clothing of a dead British officer and ends up in a German prison camp, eventually finding his way through Occupied Europe and back to England, with information for British intelligence. He finds a kindred spirit in Tommy Hambledon, who encourages him to use his position as a German agent for the benefit of the British.