This book is a longtime favorite of mine, which is worthy of a thorough re-read every few years or so. This story is magically crafted with incredible characters and a tasteful way of dealing with one of the darkest time in history: The Holocaust.
Through the portrayal of an easy-to-relate-to main character in Hannah/Chaya and a bit of "time travel", a personal experience of the horrors of the holocaust is brought to life. Yolen does an incredible job of introducing a very adult topic to a young audience. While she doesn't shy away from the violence and unspeakable situations of the concentration camps, the events are tastefully written and provide an age-appropriate challenge for kids to begin confronting some of the darker sides of human nature. Yet keep in mind that there is level of violence portrayed which must be considered appropriate on an individual basis, depending on the age and maturity of the child. I would not recommend this for most kids who are younger than 5th or 6th grade, though some younger kids may be ready to handle this content.
I highly recommend this book, for a teenager or adult. It is a wonderful choice for a classroom book to introduce the history of WWII and the holocaust or simply a easy read to begin a conversation with your child about an all-to-difficult-to-comprehend event in history.