The riddle of the sands - Erskine Childers

When Carruthers receives a letter from his friend Davies suggesting a Baltic sailing trip, the vision of a manned yacht, A1 scenery and excellent duck shooting quickly works its charm. But Carruthers hopes for a holiday are quickly dashed. There has been suspicious German activity along the coast. The Medusa, manned by the sinister Dollman, has already tried to destroy Davies. What are the Germans up to? Nothing less than a plot to invade Britain. And only these two courageous Englishmen can stop them.

Published in 1903, The Riddle of the Sands is considered the first modern spy novel.