I remember loving this book when I was my daughter's age and kept my copy for when I had my own kids. My daughter recently read it and I read it again too. She loved it, and I didn't enjoy it. I think the magic and mystery are appealing to children, as well as the description of the beautiful place that Gwen's sister goes to, but the plot is rather weak and the ending is disappointing because it leaves you without much resolution and makes the whole story rather a moot point. There doesn't seem to be much of a message behind the story, either.
However, it's a clean story, so that's a good point.
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