The Riddle of the Sands

Containing many realistic details based on Childers' own sailing trips along the German North Sea coast, the book is the retelling of a yachting expedition in the early 20th century combined with an adventurous spy story.

It was one of the early invasion novels which predicted war with Germany and called for British preparedness. The plot involves the uncovering of secret German preparations for an invasion of the United Kingdom. It is often called the first modern spy novel, although others are as well, it was certainly very influential in the genre and for its time.

The book enjoyed immense popularity in the years before World War I and was extremely influential. Winston Churchill later credited it as a major reason that the Admiralty decided to establish naval bases at Invergordon, the Firth of Forth and Scapa Flow. (Summary from Wikipedia)