This a kind of early spy novel (not super early as spy novels go back a long way) but early in the genre kind of spy novel. It was written in 1903 and takes place almost entirely on a small boat in the North and Baltic seas. Our narrator is a young sporting man from England with a background living in continental Europe and speaking German. He enlists or is enlisted by a friend to go on a sporting voyage around the seas and go duck hunting. While out they run aground several times on sandbanks along the slim coast of western Germany (Germany has very little coastline which is split by Denmark and blocked in significant ways by Poland — you recall this all from World History class of course).
As the sporting vessel is tiny and uncomfortable and clearly less useful than was promised our narrator begins to suspect some ulterior motives. Davies, the friend, admits to having deceived our narrator and tells him of a mystery he discovered after meeting a suspicious German boatsman some weeks ago. The two realizes that there’s a kind of code or puzzle being worked out along the coastlines, and they take it upon themselves to become amateur spies.
This is mostly pretty good, but it drags on both in the beginning and then at times in the middle. There’s a LOT of downtime throughout and almost nothing happens, but then becomes very interesting at different times as the plot picks up. I also like the tone a lot, and of course spy stuff is great, especially when it’s disconnected from any kind of would-be dramatic irony from this side history.