There's magic in this book, but it feels absolutely like nature. It's also essential, and in being essential becomes its own kind of character. Nature itself becomes a character, and that's a part of why the magic feels like nature.
In some ways one could consider this a moral book, because it's definitely about choices and their consequences. But it's realistic in that it acknowledges some choices might be right, but not best. Or might be dangerous, but right. Or might be wrong, but sympathetic. In other words, this book is about life, and that's why it's a "moral" book. It's as moral as life itself.
I've commented before on other books' uses of poetry, in part because I see so many unpublished manuscripts that use poetry badly. Shannon Hale uses poetry and song frequently in her books, and she does it just right. It's a part of the culture in the story, and it reminds you that the culture is different from your own while also inviting you to become familiar with it.
Both readers and writers should take a look at Shannon's blogging book club, both to learn more about the book itself, and to learn about the writing process. It's incredible to think of the journey a story takes from its first formations to its final printing, when you consider how pristine the results are. It's hard to imagine Miri's story going any other way.