The Riddle of the Sands

Written in 1903, The Riddle of the Sands was one of the earliest spy novels, one that in some ways established the espionage/adventure formula later followed by John Buchan, Ian Fleming and others. The book is the story of two young Englishmen who, while yachting in the North Sea, spot a German barge about to invade England. It's a solid thriller, with sly and witty prose.