Having studied classical Harmony, this book was a humbling experience for me to say the least. Mark Levine has really done an incredible job here explaining a very difficult subject, this is the first theory book that I have read through and understood it in the first pass. At least 50% of the material presented in this book I have not seen in any other book (and I have quite a few books).
OK there are a few things about the book that gave me a little trouble.
1. Even though this book is written for any instrument, the chord voicings are written for the piano. This is more of a problem for guitarist; you will need to know your instrument well to construct (or find an alternated voicing) some of the voicings he presents.
2. Since Mark is so specific if you have studied any theory elsewhere you are going to find contradictions in this book.
I would recommend this book to musicians with a strong understanding of music fundamentals, and a very good understanding the fret board.