The Riddle of the Sands by Erskine Childers
While on a duck-hunting holiday sailing in the Frisian Isles, Carruthers and his friend Davies become suspicious of German naval activity off the North Sea Coast. The pair decide to investigate, and are soon embroiled in a world of suspense and intrigue, and the pair set about foiling nothing less than a plot to invade England.
Initially published in 1903, The Riddle of the Sands proved a prescient vision of the Anglo-German conflict that was to culminate in the First World War. This thrilling adventure is now regarded as the first - and one of the best - spy novels ever written, inspiring later masters of the genre from John Buchan to John le Carre.