I read a lot of children's books and to be honest quite often don't even finish modern ones. There are plenty of books that are designed plainly to fit a particular reading niche, and usually these are adequate but predictable. Then there are children's books that are okay - they follow a pretty much linear plot with pretty much predictably behaving characters. But what you don't get too much of these days are books that are carefully and intelligently put together, where the characters and the plot fit so tightly that the reader can relax knowing they're in the hands of someone who knows what she's doing. Don't get me wrong - this book is not a heavy or 'worthy' read: it's absolutely gripping. But what works so well is that the development of the narrator is totally tied in with the development of the plot, i.e. one affects the other, so you are kept reading, keen to find out what's going to happen next. And you're also kept guessing about the fate of the missing Abby until the very end.
Also what's good about this book is that very profound questions about identity are raised - who we really are and who we may be trying to pretend to be in order to impress others - but always within the flow of the story, never as author-preaching.
Lee Weatherly's first book, Child X, was very good too, but this is even better. It's very heartening to see that there are still writers around who are set on improving their writing as opposed to just repeating earlier successes.