A teacher is by far the best way to go, failing that The Complete Musician is the best book/source I have seen for self study. The problem with self study is that the basics of harmony/triads seems very simple and people plow through it and think they understand than get completely stuck. Everything is built on harmony and if you do not understand it you will not progress. If you go with self study remember that if you are stuck it means you did not understand what came before, go back and figure out what you missed, which is where a teacher is very handy since they will have a much better idea about what you missed than you do.

https://www.youtube.com/@GracieTerzian is a really good teacher.

Yes, Rick Beato. He covers all sorts of stuff to different extents. Like music theory in ten minutes: https://www.youtube.com/watch?v=mWpXy57-mvc .. but he has hundreds of videos on keys, modes, analyzing popular music, the works.

If you really know nothing you might enjoy this attempt at explaining harmony at five different levels from a child to Herbie Hancock: https://www.youtube.com/watch?v=eRkgK4jfi6M

Rick Beato also has a book and interactive courses that are on sale at the moment: https://rickbeato.com/ (I have bought the bundle, but I haven't had time to check it out yet.)

An other useful channel I can recommend is https://www.youtube.com/@DavidBennettPiano

Don't feel rushed, Rick's content is always on sale. I don't have first-hand knowledge, but have read some criticisms that the "Beato Book" is not always coherent or well organized. That said, I believe he has recently made substantial revisions, and added a lot of on-line content.

Probably not the worst way to spend ~$100!

Check out Gracie Terzian on YouTube if you’re a total beginner and getting overwhelmed by other music theory sources. Her speciality is really breaking things down and simplifying as much as possible.

The other day I discovered there's actually a free college-level interactive Open Textbook on Music Theory:

* Interactive version: https://openmusictheory.github.io (v1) or https://viva.pressbooks.pub/openmusictheory/front-matter/int... (v2)

* Non-interactive downloads (e.g. PDF, EPUB): https://viva.pressbooks.pub/openmusictheory/ (via http://www.openmusictheory.com/) (v2)

The PDF I downloaded is ~1100 pages and it's the second revision of an originally crowd-funded project

I've only briefly skimmed it but it certainly seems very comprehensive & way more than I'm ever likely to need. :D

But just now I've just noticed that they describe (on https://openmusictheory.github.io/about.html) their use of the textbook for "...“inverted” or “flipped” courses, often following an inquiry-based model" and the "..text is meant to take a back seat to student music making (and breaking)".

Which matches what's motivated me to learn more about the theory side: it was after playing around & creating tracks (using the FLOSS LMMS / Carla / Surge XT music creation software) that I hit a point where I wanted to learn how to make my music have more of a melodic component to it.

And it turned out, for me, at least, creating rather than playing/reproducing music is way more motivating for me to learn the theory which I kinda wish someone had shown me during the short period multiple decades ago when my parents were paying for piano lessons until it was suggested that if I wasn't going to practice perhaps it wasn't the best investment... :)

But, you know, better late than never, I guess!

Edit:

* Added direct link to v2 interactive version.

* Re: your specific question re: pitch & tone, this seems like a reasonable starting point: https://viva.pressbooks.pub/openmusictheory/chapter/aspn/ though it also refers back to an earlier chapter: https://viva.pressbooks.pub/openmusictheory/chapter/clefs/

Rick Beato is not great for complete basics. Gracie Terzian has a really good music theory playlist on YouTube that builds on itself. I watched her videos in that order and it really helped me finally understand so many concepts in music theory that went over my head before. She has a way of explaining things that just works for me. Here’s the playlist: https://youtube.com/playlist?list=PLjpPMe3LP1XKgqqzqz4j6M8-_...

Additional recommendation that I (re?)discovered yesterday:

* "Music Theory for Musicians and Normal People": https://tobyrush.com/theorypages/index.html

It's an ongoing series of illustrated one page "posters" in PDF form each on a specific Music Theory-related topic. Nice bite-sized content written in a non-dry style. (All licensed CC-BY-NC-ND.)

Also available to download as a (currently) 63-page single file PDF.

It's a great resource if that medium of communication is effective for you--I just learned/got clarification on some things from a skim of 63-page PDF.