The Black Tower by Louis Bayard

The story of the lost king of France, Louis- Charles, who was supposedly executed after his mother, Marie Antoinette. Vidocq, an ex-convict who has transformed himself into a detective ( the world's first?) sets out to find the real Charles, aided by Hector Charpentier, son of the medical doctor who attended the prisoner Louis- Charles, and who may well have spirited him out of the Black Tower, where he was imprisoned.  A literary historical fiction thriller, well-written, and not at all predictable.  So many mysteries have only one trail, but this one has many interesting twists and turns, and also paints a vivid picture of life during the Restoration, the period following Napoleon's death.