Cricket3 beat me to The Book Thief and Number the Stars, but have you considered Schindler's List? Even if you think it might be too mature, you could look it up at Amazon and see what else pops up related.
There is a ton of material, the writing is at a post-collegiate level, and it can be somewhat technical at times, but as a single resource, it would be hard to do better than Winston Churchill's books on the subject. In particular, his first book on WWII, The Gathering Storm, is quite valuable. He describes a huge range of worldwide factors that led from one war to the next, and does so rather fairly and honestly, handing out blame to all parties. Since he was somewhat of a British Cassandra from the rise of Nazism all the way up to the invasion of Czechoslovakia, he's an outstanding source on this subject.
