It's a dear little book, written in a style similar to George MacDonald and other older writers for children; not watered down by any means, but definitely with the suggestion that good things come to good little girls and boys, and bad things come to naughty children.
I liked the story itself; the pacing was better than the movie, which sometimes felt abrupt or rushed. In the book, though, I often felt like things were falling into place TOO neatly, which is fair enough for a children's book but I guess I wanted things to be a little more complex or messy--or at least for the heroine not to triumph on her first try every time. (The one time that things did not go according to plan, she still discovered several important pieces of information which led to her eventually success.)
I hesitate to say that this might be one of the rare times when I like the movie better than the book... but it's definitely a close call, in spite of the many changes that were made to the movie. Perhaps because I saw the movie first?
The book is worth reading, especially if you like this somewhat older style of children's book. If you'd rather read a dark story with a lot of twists, you might want to give this one a pass. (Although I guess you could say there are twists in this one; to me they seemed more like things that had been subtly set up, falling into place, than surprises.)