Getting Started
If you're looking for a comprehensive one-stop shop guide on music theory, I recommend picking one of the books listed below. Take your time and read through the texts closely. Double back if you have to--these college textbooks are typically given out to students to read over the course of two or so years, so don't feel bothered if you can't grind it through in a month. If you want to get a great understanding of theory, these textbooks should do the trick.
Japanese Theory Resources
Both of these channels listed below cover music theory. Ryan Leech broadly covers general film and game music, while Gavin Leeper focuses specifically on Japanese music. I especially recommend Gavin's channel as it is a treasure trove of information about Japanese music.
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