The Turkish Gambit — Boris Akunin

It is 1877, and war has broken out between Russia and the Ottoman Empire. In the treacherous atmosphere of a Russian field army, former diplomat and detective extraordinaire Erast Fandorin stumbles upon his most confounding case. Its difficulties are only compounded by the presence of Varya Suvorova, a deadly serious (and seriously beautiful) woman with revolutionary ideals who has disguised herself as a boy in order to reunite with her respected comrade and fiancé. Even after Fandorin saves her life, Varya can hardly bear to thank such a “lackey of the throne” for his efforts.

When Varya's fiancé is accused of espionage and faces execution, however, she must turn to Fandorin to find the real culprit . . . a mission that forces her to reckon with his courage, deductive mind, and piercing gaze.