1898, Phillips Oppenheim, E. , The Mysterious Mr Sabin 

E. Phillips Oppenheim (1898) The Mysterious Mr Sabin

It was largely the success of his first spy novel, Mysterious Mr. Sabin (1898), that enabled Oppenheim to relinquish control over the family business and devote himself to a full-time writing career. “The first of my long stories dealing with that shadowy and mysterious world of diplomacy,” wrote Oppenheim in his memoirs, the novel revolves around a plot to overthrow the Third French Republic. Its central figure, Monsieur Sabin, is none other than the Duc de Souspennier, a royalist nurturing ambitions of being a new Richelieu. The stratagem he devises for ridding France of the Republic is a German invasion, promised by Berlin provided that he deliver detailed plans of British coastal defences to the Germans. [David Stafford (1991) The Silent Game: The Real World of Imaginary Spies]

Full text available at: http://www.gutenberg.org/ebooks/35661

There is a discussion of the merits, or otherwise, of The Mysterious Mr Sabin and other novels by Oppenheim in LeRoy L. Panek (1981) Special Branch : the British Spy Novel, 1890-1980