The Black Tower by Louis Bayard

Paris, 1789. A mob storms the Bastille and begins one of the most famous, and bloody, periods of French history. In 1792 the French Royal Family is imprisoned in the Temple a fortress built by the Knights Templar in the 12th Century. The following year both King Louis XVI and his wife Marie-Antoinette are executed, while their two surviving children Marie-Therese-Charlotte and Louis-Charles are locked away to either die or be reeducated.

According to official records young Louis-Charles survives for another two years before the abominable conditions of the Temple prison destroys his health and he dies from tuberculosis at the age of ten. His body is then placed in an unmarked grave and promptly forgotten.

At least it was until a cryptic message found in a dead man’s pocket point medical student Hector Carpontier and noted detective Eugène François Vidocq on the trail of a mystery that reveals what many have long suspected that Louis-Charles, also known as Louis XVII, the rightful king of France is indeed alive.