I was fortunate to be at the University of Pennsylvania from 1986-89, when Dr. Narmour was working on this book. He was extraordinarily generous with his graduate students, in that he included us in the process of working out the ideas in this book. Penn has always been an iconoclastic school where music theory is concerned -- rejecting Schenker's religious and racist views of how "superior" music is structured ("superior" meaning that it reflected the "trinity" and the ideals of Nazi culture) -- and this work takes that view to the opposite extreme, arguing that while cognitive understanding of what is happening in a piece of music is universal, the true understanding of a piece of music requires an understanding of both cultural and chronological placement.
While Schenker argued that any piece of music could be boiled down to the same background structure ("three blind mice", in essence), Narmour agues that the value in any piece of music is how it differs from others. This cultural and chronological neutrality of the theory -- that is, it argues no superiority of one culture or period over another -- is what makes this approach to analysis so attractive -- and is also the reason that Narmour has been so disparaged by the musical establishment at Yale, Columbia, and the like, who still teach Schenker as if it were gospel.
This is NOT easy stuff. But I believe that it will stand the test of time.