Anthony Doerr’s new novel is the story of two children: a blind French girl who flees Nazi-occupied Paris in 1940 to live with an eccentric great-uncle in the walled city of Saint-Malo, and a German boy whose talent for fixing radios earns him a spot at the National Political Institutes of Education, a training school for Hitler Youth. Linking their stories is a 133-carat jewel thought to bring ill luck.