In March 1942, a young Jewish-Slovakian refugee embarked on a two-year journey that would rival Dante’s descent into the Inferno. As a slave laborer at Auschwitz, Walter Rosenberg experienced starvation, endured torture and witnessed savagery that even Dante could not have imagined. Throughout, he also observed, and committed to memory, details of every aspect of the Third Reich’s machinery of mass murder, down to the number of men, women and children—as well as their dates of arrival and places of origin—who were deposited by train en masse at the Auschwitz entrance ramp, and through whose gates almost all were ushered...