The Black Tower by Louis Bayard

Vidocq! Master of disguise and chief of a newly created plainclothes police force, Vidocq is a man whose name sends terror rippling through the Parisian underworld of 1818—and the inconsequential life of Hector Carpentier is violently shaken when Vidocq storms into it. A former medical student living in his mother's Latin Quarter boardinghouse, Hector finds himself dragged into a dangerous mystery surrounding the fate of the dauphin, the ten-year-old son of King Louis XVI and Marie-Antoinette presumed to have suffered a cruel death years earlier in Paris's dreaded Temple. But the truth of what happened may be even more shocking—and it will fall to an aimless young man and the most feared detective in Paris to see justice done for a frightened little boy in a black tower . . . no matter what the cost.