Cenotaph for the Living - The French Collection, #2

In France, since 2013, the authorities will no longer search for adults who have been reported missing by family members. Approximately 15, 000 people disappear each year without trace. Tristan Gautier has just become one of them and, when his mother receives a kidnap demand, it has come from someone in the commissariat de police near to the cemetery of Père Lachaise in Paris. Having paid the ransom demand, Tristan is still help captive and the kidnappers make it clear they will kill their hostage unless they are paid even more money.
The threat is real and Tristan's father plans to find where he is being held. But the plan falls apart when shots are fired. The only clues the family have now to Tristan's whereabouts come from a man who has been dead for 150 years. To discover where Tristan is, they must try to unravel the secrets of the man's identity from just a handful of cryptic clues. Can Tristan last out that long or will the kidnappers dispose of him first?