I nominate
The Great Impersonation by E. Phillips Oppenheim. Here's a blurb from Goodreads, where it has 3.92 stars from 518 ratings:
Quote:
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The Great Impersonation is probably the most famous spy novel of all time. Allen Dulles put it at the top of all spy novels for its virtuosity. It is marvelous reading with its fast-moving plot, its descriptions of the rich life of English aristocrats before the Great War, and its bold characters. Besides the Kaiser and a whole host of Dukes, Duchesses, Ambassadors, German agents and silly young Englishmen, there's the Princess Eiderstrom, "one of the most passionate women in Europe," desperately in love with Leopold; Sir Everard's insane wife who has vowed to kill him if he should ever return home; and a rollicking finale.
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Best of all, since it was published in 1920 by an author who died in 1946, it's public domain in both the US and Life+70 (and less) countries. It's free in lots of places; I'll link to the
ePub here at MR and the
Kindle edition at Amazon.