
Harmony by Walter Piston (and also "Twenty First Century Harmony," the book it inspired) is IMO one of the better books I've read on western music theory. I got my copy used for $20.
SICP is nice to loan out (and also nice to read, it covers some pretty weird and advanced topics near the ends of the chapters), my girlfriend is going through my copy. ~$20.
A couple Oreily books for whatever you're doing at work ~$120 (although double check, believe it or not there are a couple duds.)
UML distilled by Martin Fowler is very good. ~$20?
The art of electronics is a great book, I personally learned a lot from this kind of weird book titled "Basic Pulse Circuits" I found at a used book store for ~$10.
Then leave the extra $100 for whatever interesting thing you find at good book stores (I've had the best luck in college towns in Appalachian Virginia.)
I think the only thing you’ll need to know going into it is how to read sheet music.
It doesn’t really go too much into counterpoint or melody if I remember correctly, for those there are couple YouTube channels I ended up watching that are pretty much just a narrator analyzing other people’s music:
1) 8bit music theory
2) Richard Atkinson
There are others but those are my favorites.
The Jazz Theory Book (Mark Levine)
Huge series of play-along books by Jamey Aebersold
The Geometry of Musical Rhythm (Godfried Toussaint)
Harmony (Walter Piston)
A Geometry of Music - Harmony and Counterpoint in the Extended Common Practice (Dmitri Tymoczko)
Lydian Chromatic Concept of Tonal Organization (George Russell)
Augmented Scale Theory (Javier Arau)
This list has a recommendation for Introduction to Tonal Theory over Piston's Harmony. I am not familiar with the former, but I found Walter Piston's books to be fascinating and generally very good. It's an older book and so, as I've found with most textbooks from its time, it is somewhat lacking in explanations of certain examples– but then, the internet will supply you with more than you need if you're already reading the material.
