For reference, a list of noteworthy texts on music theory:
Big step forward. Rameau was the first theorist to frame Harmony in terms of "triads" which have "roots." But besides this huge development, a lot of his ideas are contradictory, arbitrary, and sometimes even a little incomprehensible (it's possible I was just reading a bad translation).
Riemann's Treatise seems to be where the idea of "Negative Harmony" originates, that mind virus which has recently been popularized by Jacob Collier. For this reason, alone, Reimann is an important read. It can be really clarifying to study the origins of these really bad ideas. The more one reads this Treatise, the more it becomes clear that the concept of "negative harmony"/"undertonality"/"utonality"/whatever was only thought up in order to justify the existence of the "minor triad" and to frame a polarity between it this triad and the "major triad." Riemann was (like many of the German "intellectuals" of this time period) more concerned with justifying the superiority of German culture than trying to understand it. All of his ideas can be regarded as a shallow aggrandizement of der German style of Musik.
The first work on "acoustics" and, more importantly, the beginning of a scientific branch of music theory. The theory of Consonance that Helmholtz presents is based on overlapping harmonics: tones "fuse together" because they share harmonics in certain places, and this mitigates "beating." But it's worth noting that this theory can only really be applied to tones with normative harmonic-spectrums. If we follow Helmholtz's logic, then tones with fewer harmonics should be less consonant with each other than tones with more harmonics. But the opposite is true.
The most ratio-brained theorist of them all. Partch set out to dismantle a lot of the arbitrary rules and systems set in place by academic theorists, but in the process he just ended up leaning farther into the big arbitrary rule set up by Pythagoras: "Consonance = simple-number ratios." He even outwardly states that thinking about Harmony is "thinking in ratios." His theory of Consonance is really nothing more than a restatement of Pythagoras. In my opinion, this pre-occupation with the ratios, has damaged the Microtonal movement, which Partch is considered to be the founder of, in its crib.
God I hate the Neo-Riemannians.
Uses Helmholtz's "overlapping harmonics" theory to create a quantitative model of the Consonance and Dissonance of ratios. I have already explained the problems with both Helmholtz's theory and the ratio-centric view of Harmony.