Takeaway: History and heritage matter.
One of the nice features of the Bruno, Chief of Police series is the mixing in of real history, culture and geography along with the fiction.
In this case, Walker is mixing two pieces of real history into the mystery.
The first is the largest train robbery in history. In 1944, the French resistance stole what would be the rough equivalent of about $400 million dollars (or to the comparisons in the book, 5 times the total education budget of France that year or 5% of the total French national budget.)
One of the last resistance fighters still alive in St Denis (and one of the members of that train robbery) died. Because he died with two of the bills from the train robbery in his hand, Bruno starts investigating his connection.

Summary: Finding spiritual practices for those that have become dead to spiritual practices.

Summary: Bruno looks into a shady community market and stumbles on a series of international crimes.
Summary: An arson on a remote (and illegal) genetically modified research farm leads to a crisis. And it might be related to a potential new investment by a large winemaker. Bruno as chief of police and lover of his small town seeks to preserve the community.
Summary: The children’s classic story about running away, living in a museum and solving a great mystery all the while finding the important things in life.