Both George Orwell's Nineteen Eighty-Four and Ray Bradbury's Fahrenheit 451 would make strong companion books on the themes of defiance, censorship, and the power of words and propaganda.
WWI veteran Erich Remarque's All Quiet on the Western Front, written in the late 1920s and banned by the Nazis, focuses on the futility of war and also features a German protagonist.
Holocaust non-fiction such as Elie Wiesel's Night can bolster a unit on the Holocaust and invite discussions about the different experiences of Jews and non-Jews in Nazi Germany. Similarly, Anne Frank's Diary is an obvious companion to Liesel's story.
For older (16+) students, The Book Thief's...