I think I've mentioned several times that we were studying Mozart in January because we were supposed to see a kid's version of The Magic Flute, but we missed it because of our viruses. We really enjoyed the story of The Magic Flute and particularly enjoyed a book called Sing Me a Story: the Metropolitan Opera's Book of Opera Stories for Children. We read 2 chapters of this in bed while I was sick. We read both the story of The Magic Flute and Hansel and Gretel, which we saw performed last year.
The stories in the book are fairly short and have some illustrations. But, what we really enjoyed, was acting out these operas. We got various teddy bears, dolls, etc and assigned them roles to act out. Alexandra also made props out of pony tail bands and tissue. It was a lot of fun and made the stories more "real" and memorable.
There's also a book by the same author called Dance Me a Story: Twelve Tales from the Classic Ballets. I'm adding both of these books to my wish list!