Reddit mentions of In the Process of Becoming: Analytic and Philosophical Perspectives on Form in Early Nineteenth-Century Music (Oxford Studies in Music Theory)

Schenkerian analysis takes everything you learned in harmony and makes it useful. It helped me make sense of counterpoint too. I link some sources in this thread.

Then there's neo-Riemannian theory, which is more or less about chords that don't behave well under traditional analysis. I learned from bashing my head against really dense academic articles, but I hear Richard Cohn's Audacious Euphony is supposed to be good. What you want to do is learn PLR-family transformations and the triadic Tonnetz, learn the concept behind compound transformations, then jump into Cohn's writing on cyclical progressions and wrap your head around Douthett & Steinbach's graphs.

Form is super important, especially Caplin's theory of formal functions. This stuff meshes well with Schenkerian theory, in my opinion. Schmalfeldt's book is also a very useful for the study of form. I always feel a little strange recommending Hepokoski and Darcy, because that book is so dense and I don't want to push anyone into purchasing a 600-page book on the analysis of sonata forms that's just going to end up collecting dust on their shelf. However, it's easily one of the most important music theory treatises of the 21st century so far (and the other ones I'm mentioning are right up there too), so I feel I should mention it.

Question is, what are you into? What do you want to learn? What do you hope to do with this knowledge?