This is a first class children's novel. I had hopes for this book, but it far exceeded them. The writing is excellent, as is the world building. The plot is clever and well written, and the language is delightful. The individual sentences are a delight to read. Like all goods children literature it offers something to both adults and children and has meanings on multiple levels. It is also remarkably inventive.
What is most striking is the stunningly good characterization. The book very surely draws nuanced and complex characters quickly and effectively. No one is a stereotype and everyone has quirks and subtleties to their characters. The heroine is a compelling and charming character who spends virtually the whole book turned into an old lady, but is absolutely mesmerizing, although quirky and flawed.
The book touches on a great many themes, is amusing and charming, sophisticated and subtle. It is a real find and is far superior to much better known children's works. I can honestly say that it is one of the best children's novels I've ever read and the best I've read for the first time in many many years. It is also a reasonable candidate for the best book of any kind that I've read this year.
I can't recommend it highly enough.