Unfortunately, to understand all of this book you have to have read through a good deal of books to understand this, including some works by Homer, Egyptian Mythology, and several books by Lloyd Alexander. There are far more, but unfortunately I don't have it in front of me right now so I can't list them here.
However, if you don't compare it to the other books, than it is really very good. The earlier books in the series set a bar that this one just barely didn't reach. On the upside, this book requires less knowledge of literature to understand.
The plot is a little more complicated than in previous novels, and may take some rereading to understand fully, but overall it is just as good as its companions in the series. (Though the talking badgers, I think, are a bit much.)