After two years hiding from Nazis behind a false wall in the attic of a flour mill in Tarnow, Poland, seven-year-old Israel Unger developed a strange habit. For most of those two years, the nine people crammed into that small space had eaten the only thing they’d been able to get. That was a basic bread made of flour and water and a “soup” made of hot water and the barley that Unger’s father managed to sneak into the room — along with the flour — from the mill downstairs. When young Israel got his soup in the evenings —...