If your novel is set in WWII France, this book will help you avoid mistakes, perhaps small ones like your Frenchman breakfasting on bacon and eggs or more serious like confusing the various branches of the occupying powers, mistakes guaranteed to knock the shine off your bestseller.
I hope that my bringing together in one handy volume details of life in France during WWII, you, as an author, will have an authoritative reference to help in your research.