This Light Between Us-a novel of World War II

In 1935, ten-year-old Alex Maki of Bainbridge Island, Washington, is horrified to discover that his new pen pal, Charlie Levy of Paris, France, is a girl, but in spite of his initial reluctance, their letters continue over the years and they fight for their friendship even as Charlie endures the Nazi occupation and Alex leaves his family in an internment camp and joins the Army.