I dunno. On first glance, for a "21st-Century" textbook, this seems to place a high emphasis on classical music theory, and moves very highly influential modern styles to the back burner -- notably jazz. And there is essentially no discussion of music theory in other cultures (middle eastern, african, east asian, indian), nor current analysis of pop and rock music.
I know a lot of people with music degrees. This is a widely acknowledged problem. It doesn't have a simple answer, especially as academic music programs are shrinking. You can't teach everything except at a survey level. You have to find faculty who can teach new topics at a college level. And students who are prepared to dive in. Music students tend to already be damn good musicians by the time they start college.
If programming is an analogy, it's said that once you've learned your first language, you can learn any other language in a jiffy. Also, a lot of higher level learning is "learning how to learn." Maybe it's better to have a standardized "first language" that students can learn in depth, and then let them move into other areas at their own discretion.
I don't have a college degree, but for me, classical music lessons were a great preparation for learning jazz, and I've picked up pop, rock, and folk music on my own. In fact, on the instrument that I play, the double bass, gaining proficiency and experience in jazz without that background is a brutal learning curve.
> And there is essentially no discussion of music theory in other cultures (middle eastern, african, east asian, indian)...
To me, that above ^ looks like the biggest gap.
To be fair, the book doesn't claim to demonstrate theory of the "world music" (sic, a phrase as bad as it sounds).
My (brief and shallow) understanding is that most scholars of music in the western world just do not have enough knowledge to talk intelligently about non-western genres. Exceptions exist, of course, but they are quite rare.
[Other reason is that there is a lot of gate-keeping. I've seen that in Hindustani/Carnatic classical studies, FWIW, which is one my areas of interest]
"Oh no, math degree... Where is 2+2=4?"
I have a BA in Music Theory, I think maybe you should consider again. First off, Jazz relies on the same fundamentals and extends them, everything from Classical music is somewhat relevant in Jazz. So if it's only Jazz theory exclusive to classical, it'd be an advanced book indeed.
Second, it literally has a chapter on Popular Music and another on Jazz Theory, and "Classical" is not found in its chapter titles. So I don't think your critique is accurate.
Third, history and time matter. "Theory" came first in "classical" contexts, with codified rules pre-dating even Bach. Jazz was a much later invention that built upon these and other traditions. Both of the Jazz theory books I used in my BA in Music Theoryhad many elementary topics already covered in Classical contexts.