On October 30, 1942, a group of destroyer warships from the British Royal Navy hunted down a Nazi submarine near the Nile Delta. The warships pounded the submarine with underwater explosions until it floated to the surface, where it started filling with water and sinking. As its German crew scrambled to escape, three British heroes—Lieutenant Anthony Fasson, sailor Colin Grazier and 16-year-old canteen assistant Tommy Brown—did something that defied all instinct. They jumped from their ship onto the sinking vessel and climbed inside. They were after the sub’s most valuable cargo: not weapons, not prisoners, but books. The pages contained...