[quadrangle]

Just Intonation and everything from https://en.xen.wiki/ and related, maybe some of the YouTube stuff from "Hear Between the Lines" is a good start for new accessible stuff.

Music cognition such as Sweet Anticipation by David Huron and everything like that… especially note Music and Memory by Bob Snyder which expresses everything except harmony in a style that avoids traditional notation, and Sounds of Music: Perception and Notation (out of print) by Gerald Eskalin who also wrote Lies My Music Teacher Told Me

For the most part, "music theory" amounts to music-notation-and-style-grammar because very little of it is actual theory (i.e. explanation) until you add real science which means music psychology (because notation patterns and physics patterns are not music, music is a mental experience).

[mkl]

Looks like it's "Eskelin".