In the Garden of Beasts by Erik Larson
From Wikipedia: "The book covers the career of the American Ambassador to Germany, William Dodd, particularly the years 1933 to 1937 when he and his family, including his daughter Martha, lived in Berlin. The Ambassador, who earned his Ph.D. in Leipzig 40 years earlier, was initially sympathetic to Germany's new Nazi government and believed reports of brutality and anti-semitism to be exaggerated. Martha, separated from her husband and in the process of divorce, became caught up in the glamor and excitement of Hitler's Germany and had a series of liaisons, possibly sexual, including among them Gestapo head Rudolf Diels and Soviet attache (and secret NKVD agent) Boris Vinogradov. She defended the regime to her skeptical friends. Within months of their arrival, the family became aware of the evils of the Nazi party. Dodd periodically protested against it. President Roosevelt was pleased with his performance while State Department officials found him undiplomatic and idiosyncratic."