Traitor's Gate by Michael Ridpath
Arriving in Nazi Germany, Englishman Conrad de Lancey feels sick at heart. Even Berlin - infamous haven of decadence and vice - salutes fascism. Himmler's black-shirted troops rule the city, and every German arm bears a Swastika. But does every German heart belong to Hitler?
When Conrad is arrested by the Gestapo on suspicion of spying, he is rescued by Theo, an old friend from university, now a lieutenant of the Wehrmacht. Together they are drawn into a world of danger and deceit, of plots, paranoia and intrigue where the brave few are united by a single ambition: to free the fatherland from the Fuhrer.