This is something I had mentioned on the old forum. The resource is rather unconventional as it doesn’t use musical notation at all, but it is remarkable in the way it covers its contents. It makes you think and sometimes re-examine your approach to music. Things that I was vaguely aware of became much clearer. I also found new things that had never been explained in any other resources I’ve come across.
Having said that, you may find some small inaccuracies or sometimes even disagree with the author. For instance, his circular harmonic scale chord progression chart is very useful, but he claims that a lot of musicians mistakenly think that the Circle of Fifths has something to do with chords and chord progressions. Well…, that’s a matter of interpretation.
Nevertheless, I think it’s a resource worth looking into. It’s available in both digital and print formats. You can read about half of the contents online for free. Here’s the link to How Music REALLY Works!