This excellent 2015 book sold over 4.5 million copies worldwide before I read it.

Kristin Hannah has become one of my favourite writers.

… the story of two sisters in France during World War II and their struggle to survive and resist the German occupation of France.

It was inspired by the story of a Belgian woman, Andrée de Jongh, who helped downed Allied pilots to escape Nazi territory. …

wikipedia

Dakota and Elle Fanning will star in the planned movie as the sisters — which would be excellent casting, IF they were French. They were born in Georgia.

This book reminded me of Beneath a Scarlet Sky by Mark Sullivan, the story of a young Italian man helping Jews escape over the Alps.