3.5 stars.
Everything I know about music I learned from this book. It is very thorough. It is also quite technical and does not make for light reading. It took me a long time to finish it - like, over a year - and I read several chapters more than once.
I also own two editions of this book: a physical copy of the first edition (2005), and a digital copy of the third edition (2016). In terms of organization, the third edition is an improvement, but still not perfect, for reasons I will explain later. I also bought the CD set for the first edition and a CD player, which I do not recommend: there are 3 CDs, each with 90+ tracks, and they are not arranged in the same order as they appear in the book. So there is a lot of searching for song clips that are, in some cases, only a few seconds long.
I think the book teaches the basics (notes, scales, chords, etc), counterpoint, harmony / tonality, and chromaticism well but teaches forms and modernism less well. Even where it succeeds, I don't think it is a great standalone resource. (I ended up watching a lot of YouTube videos, for example.)
Some suggestions, for the team at W.W. Norton & Co:
1. Focus on the common practice period, merge redundant chapters, and trim the fat. When it comes to technical writing, economy and clarity are best practices.
2. Arrange the chapters into smaller units that better describe the content. The third edition divides the middle 750 pages into two units about diatonic and chromatic harmony. Really? The distinction strikes me as a minor one, because these chapters cover a lot more than that.
3. The absence of overtones and temperament is a shame, and I think it would be advantageous to include these topics before the others. Understanding consonances, dissonances, and resolutions is much easier with this foundation - but, as a practical matter, I recognize that rooting music theory curricula in acoustics and physics may be enrollment suicide.