How did German intelligence agents use a dead fish to convey critical information to their operatives? What did an advertisement for a dog in the Times have to do with the movement of British troops into Egypt? And why did British officers suddenly become suspicious of the trousers hanging on a Belgian woman's washing line?

Throughout World War I, spymasters and their networks of secret agents developed many clever--and sometimes comical--methods of covert communication. Stacks of bread in a bakery window, puffs of smoke from a chimney, and even woolen pullovers were all used to...