I nominate In the Garden of the Beasts by Erik Larson. I just downloaded this from the library and am eager to read it. It looks very good, and this is from the same author as Devil in the White City, which was also excellent.
In the Garden of Beasts is a vivid portrait of Berlin during the first years of Hitler’s reign, brought to life through the stories of two people: William E. Dodd, who in 1933 became America’s first ambassador to Hitler’s regime, and his scandalously carefree daughter, Martha. Ambassador Dodd, an unassuming and scholarly man, is an odd fit among the extravagance of the Nazi elite. His frugality annoys his fellow Americans in the State Department and Dodd’s growing misgivings about Hitler’s ambitions fall on deaf ears among his peers, who are content to “give Hitler everything he wants.” Martha, on the other hand, is mesmerized by the glamorous parties and the high-minded conversation of Berlin’s salon society—and flings herself headlong into numerous affairs with the city’s elite, most notably the head of the Gestapo and a Soviet spy. Both become players in the exhilarating (and terrifying) story of Hitler’s obsession for absolute power, which culminates in the events of one murderous night, later known as “the Night of Long Knives.”