(The Guernsey Literary and Potato Peel Pie Society- Mary Ann Shaffer & Annie Barrows) Great historical fiction about the island of Guernsey during WWII. Written completely in letters, which in this case, totally works.
Night - Elie Wiesel This is also one of my favorite books to teach. It's about a father and son trying to survive the concentration camps during the Holocaust. My students and I wrote to the author this past school year and he actually wrote back to us!