If you can get your hands on a copy, A Very Young Dancer is my favourite ballet book ever. It tells the true story in words and photos of a young girl who dances at the Juilliard School in New York - it culminates with her playing the lead child part in The Nutcracker with the New York City Ballet. The photos are pretty old school now (it was published in the late 70s) but it's mesmerizing stuff. I still sometimes look at my copy!
ps You can find it quite cheap through bookfinder.com.
Ballet School by Naia Bray-Moffatt. My dd loved this one when she was that age. It sounds a bit like Bue's one, but with photos from pupils at the Central School of Ballet in London, shows what happens at ballet school.