The Ministry of Fear by Graham Greene

For Arthur Rowe, the trip to the charity fête was a joyful step back into adolescence, a chance to forget the nightmare of the Blitz and the aching guilt of having mercifully murdered his sick wife. He was surviving alone, outside the war, until he happened to win a cake at the fête. From that moment, he is ruthlessly hunted by Nazi agents and finds himself the prey of malign and shadowy forces.