The Riddle of the Sands, published in 1903, is one of the great classic spy stories and at the time it was written there was considerable overlap between crime fiction and spy fiction (with Sherlock Holmes himself crossing swords with agents of foreign powers on several occasions). You could call it a spy mystery.
It’s also a great adventure story and a very fine novel of the sea. It concerns two young English yachtsmen who become obsessed by the idea that Germany is up to something nefarious in the waters just off their North Sea coast, a region of constantly shifting sandbanks and treacherous and changeable channels. To say any more about their suspicions would spoil the story.
This is low-key spy fiction. There’s not a great deal of action. The emphasis is on a gradually building tension as the full significance of what at first appears to be fairly tenuous evidence is slowly revealed.
At the time it was written Britain and Germany were engaged in a frantic naval arms race. This was the first real threat to Britain’s naval supremacy for a century, resulting in rampant paranoia about the possibility of German plans for invasion in the event of war. And paranoia is the main ingredient here as our amateur spies realise they’ve uncovered something incredibly important but that it’s going to be very difficult to convince anyone in authority. They’re going to have to do the investigating themselves.
The author, Erskine Childers, was an interesting character in his own right. Although The Riddle of the Sands was very much a stirring story of English patriotism Childers himself was executed by the British during the Irish Civil War in 1922.