The Riddle of the Sands by Erskine Childers (1903)

This early spy novel is considered a classic of the genre. Written in 1903, it tells the story of two young English yachtsmen who stumble upon the fact that a fellow countryman has most likely allied himself with the Germans in planning for warfare against the British. In an effort to find actual proof that they can convey to authorities they undertake many risky escapades, both on land and sea.

The writing is typical of the period in which it was written, formal and not the easiest to read. Complicating the readability, Childers spends a great deal of the novel in detailed reporting of the men's maneuvers in their sailing yacht, complete with technical and nautical terms, which are essentially meaningless to a landlubber. But when Childers gets his characters on land, the tale becomes quite suspenseful and exciting.

The Riddle of the Sands was written more than a decade before the start of World War I and is credited with alerting the public to the possibility of German aggression and is reported to be partly responsible for the British strengthening of their sea defenses. It is also an interesting adventure yarn.