Yes, it's a great book.  You're right that it helps us to understand how to read Scripture.  I think it's also a great example for other biblical scholars of how to write sound biblical theology in a way that is accessible to a general audience.  'The Birth of the Messiah' is really two books: a commentary on the Infancy Narratives which can stand on its own; an scholarly notes that support the commentary.  The notes are rewarding for those who want to go deeper, but they are not necessary for following the commentary.  The book is a beautiful combination of academic scholarship and pastoral insights.