I'd like to nominate
The Riddle of the Sands, by Erskine Childers.
From Goodreads:
Quote:
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While on a sailing trip in the Baltic Sea, two young adventurers-turned-spies uncover a secret German plot to invade England. Written by Childers—who served in the Royal Navy during World War I—as a wake-up call to the British government to attend to its North Sea defenses, The Riddle of the Sands accomplished that task and has been considered a classic of espionage literature ever since, praised as much for its nautical action as for its suspenseful spycraft.
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Published in 1903 and Childers having died in 1922, it's public domain everywhere but Mexico, I'm guessing. There are two versions at Mobile Read: in
epub and in
LRF, the latter uploaded by HarryT.