We’ve been studying Mozart. So far, we’ve enjoyed several books I wanted to recommend.
Anne Gatti’s The Magic Flute is a retelling of the opera’s story, with an accompanying CD with selections from the opera. My children enjoyed it a lot.
We also enjoy the Classical Kids CDs, and the Mozart one is fun. 
Young Mozart (Rachel Isadora) and Mozart Finds a Melody (Steven Constanza) are very accessible picture books about Mozart. For more depth, we read the biography The Life and Times of Mozart (John Bankston) in the Masters of Music Series.
Learning As We Go 