When Hitler's armies occupied Italy in 1943, they also seized control of mankind's greatest cultural treasures. As they had done throughout Europe, the Nazis could now plunder the masterpieces of the Renaissance, the treasures of the Vatican, and the antiquities of the Roman Empire. In May 1944 two unlikely American heroes--artist Deane Keller and scholar Fred Hartt--embarked from Naples on the treasure hunt of a lifetime, tracking billions of dollars...