I recently found a copy of Ballerina at a used bookstore and I read it a few weeks ago. Stewart's writing "style" is horribly ridiculous purple prose, very melodramatic, but I agree that the story is interesting and compelling. I really got to care about the characters, too. It's about 500 pages long but an easy read; I read it in two days.
I can't believe I forgot to mention this one before: On Stage, Please by Veronica Tennant. It's a favourite of mine, and it's in print. It's about a ten-year-old girl named Jennifer and her first term at the National Ballet School. It's hardly surprising that the details of the ballet world, and ballet school world, should ring true, considering its author. As an added bonus, Tennant is a talented writer. Great book.
Although I never read them when I was a kid, I'm now working on acquiring the Satin Slippers books. I've bought three of them so far-- #2, #3, and #6, and I know there are copies of five others in a used bookstore downtown, but I haven't purchased those yet. I haven't read any of them yet-- I've got plenty of reading to do for school, too! :shhh: