Erskine Childers, The Riddle of the Sands ****

The Riddle of the Sands is advertised as an "early example of the espionage novel," which is it, but it's also a sailing story, especially in its early chapters. Fans of action will find the story too slow moving. The characters spend weeks navigating the shallow waterways of the Frisian Islands trying to solve a mystery, but I was more intrigued by the navigational techniques than the mystery... until the last few chapters. The plot kept me guessing until the end. 

I enjoyed The Riddle of the Sands much more than other early spy novels that I've read, namely Conrad's Secret Agent and Chesterton's The Man Who Was Thursday.