This book is a mess. It was first published in serial form under the title “Le chercheur des trésors” for the magazine Le Matin, and this shows in the repetitions and reminders to the readers. The treasure hunt was a marketing gimmick for the paper, which apparently hid clues to prizes cached in Paris in the series instalments. At any rate, the treasure mentioned in the first chapters is completely ignored until chapter 26, and ignored again after its finder is killed.
So what is this about? Naïve, inoffensive Théophraste Longuet has retired from his rubberstamp business and wants to educate himself in the history of Paris, so he visits the notorious Conciergerie prison with his wife and best friend. They soon notice a strange change in his behaviour, and it becomes obvious that a second soul inhabits Théo’s body: the famous 18th century bandit Cartouche. They seek help from a noted spiritualist, but the criminal proves hard to shake, and poor Théo even finds himself on an extended stay in the subterranean fields of Paris among a strange people, the Talpa.
The author as journalist pretends to build his story from a variety of papers written by different people and offered to him by Théo’s best friend and executor. The style of the various sections changes with every witness-contributor, commented with a sharp and often nasty pen by the compiler. There’s a “psychic surgery” intended to get rid of Cartouche, a truly hideous description of Cartouche’s torture in the dungeons of the Conciergerie which also affects Théo, more nastiness whenever Cartouche’s soul takes over and reenacts his crimes in modern-day Paris, and then the final, absurd episode when Théo and police commissioner Mifroid drop into the catacombs and Mifroid spouts the latest scientific fads much in the manner of Dickens’ Mr Gradgrind. The story remains an episodic hodgepodge, and some of the scenes need a strong stomach. The author also exercises a cruel wit at the expense of his characters. It left me wondering what Leroux had been smoking or drinking when he concocted this.

ETA: The ebook was very carefully formatted and transcribed. ( )