Diary of a dead man on leave - David Downing

In April 1938, a man calling himself Josef Hofmann arrives at a boarding house in Hamm, Germany, and lets a room from the widow who owns it. Fifty years later, Walter Gersdorff, the widow's son, who was eleven years old in the spring of 1938, discovers the carefully hidden diary the boarder had kept during his stay, even though he never should have written any of its contents down. What Walter finds is a chronicle of one the most tumultuous years in German history, narrated by a secret agent on a deadly mission.