The Riddle of the Sands by Erskine Childers

The Riddle of the Sands: A Record of Secret Service is a patriotic British 1903 novel by Erskine Childers.
It is a novel that "owes a lot to the wonderful adventure novels of writers like Rider Haggard, that were a staple of Victorian Britain"; perhaps more significantly, it was a spy novel that "established a formula that included a mass of verifiable detail, which gave authenticity to the story – the same ploy that would be used so well by John Buchan, Ian Fleming, John le Carré and many others." Ken Follett called it "the first modern thriller."