An Introduction to Tonal Theory

This book is an excellent book on what exactly is going on in music, and why what leads to music that our brains are capable of parsing as music. It discusses the structure of melody, and makes the spicy claim that harmony is all bullshit (arising only due to counterpoint, but not being worth studying on its own.) Melodic lines have structure due to how our brains want continuity, and Westergaard discusses tools for maintaining or breaking that desire of continuity depending on the goals. He gives machinery for parsing music into these underlying operations, or, alternatively, a set of rules for deriving music from basic structure, as well as providing an “interesting-to-humans” metric that closely correlates to the ambiguity and depth of that parse.

I don't know if this book has made my musical skills any better, but it's far and away, hands down the best book I've ever read on music theory. If you've ever been frustrated by the wishywashiness of usual theory that depends on memorizing a billion facts without giving any explanation as to where those facts came from, this is the book for you.