One of the things I've enjoyed about Sharon Creech's novels is the way that she takes several seemingly disconnected threads of her story and weaves them together into a complete tale that makes perfect sense, once seen from the right perspective. They also have a habit of coming full-circle, so that it's almost like looking at a Möbius strip, and in The Castle Corona, the author doesn't disappoint in this respect.
The thing of it is, when one is more attuned to the fact that this is how the author writes her stories, one begins to be able to successfully predict large parts of the story. It spoils the surprise, and makes the story less effective. In addition, the characters in The Castle Corona aren't as interesting as other characters that she has created for other books, so it's a little harder to get into the story to begin with. They're still likable, and I think that readers will still care about what happens to them, but there just seems to be something missing. Maybe it's just the characters themselves; not a lot happened in the book, and the characters seemed to develop only a tiny bit by the end of the story. They just leave me unsatisfied, as if there were something big and huge in their lives just waiting to happen, and then the story stops just before that thing happens.
There are major events that take place, but they're few and far between, and their impact, while important, doesn't seem to have a truly lasting effect. Ultimately, I think the book's wasted potential is what frustrates me the most. I know that the author can write moving, important stories, and I even see hints of it in The Castle Corona. Knowing that she could, but didn't, is what bothers me the most about this book.