The Riddle of the Sands by Erskine Childers

While on a sailing trip in the Baltic Sea, two young adventurers-turned-spies uncover a secret German plot to invade England. Written by Childers—who served in the Royal Navy during World War I—as a wake-up call to the British government to attend to its North Sea defenses, The Riddle of the Sands accomplished that task and has been considered a classic of espionage literature ever since, praised as much for its nautical action as for its suspenseful spycraft.