Riddle of the Sands by Erskine Childers

One of the first great spy novels, The Riddle of the Sands is set during the long, suspicious years leading up to the First World War. In spite of good prospects in the Foreign Office, sardonic civil servant Carruthers is finding it hard to endure the boredom of his life in London. He accepts an invitation from a college friend, Davies, a shyly intrepid yachtsman, and joins him on a sailing holiday in the Baltic, and there, amidst the sunshine and bright blue seas, they discover a German plot to invade England .