Rockschool Music Theory
From Rockschool's website: "The Benchmark for Accomplished Musicians. Performance is a huge part of being a musician, but we believe it is a firm grasp of musical language that opens up the door to achieving your full potential. Our goal is to aid the development of articulate, accomplished musicians, and the Popular Music Theory resources have been specifically designed with this in mind."
I have always been interested in the little black dots. It just took me a while to get the time to study them. Even then, not coming from a classically trained background, it was tricky. Now that I know a little bit more about them, I find it absolutely imperative that every musician who wants to play with other musicians should at least have a very basic understanding of music theory. It makes communication with other musicians so much easier and more efficient. It makes composing more enjoyable. It makes writing down your thoughts more comprehensible.
I have been teaching the Rockschool Popular Music Theory Syllabus since 2015. They have been the leading rock exam board since 1991, and with the introduction of their Music Theory Syllabus they expanded their subject matter to include a course that imparts essential knowledge to popular music instrumentalists. I wish I had the Rockschool Popular Music Theory Syllabus when I started playing music. It is focussed on the modern popular musician and styles like Rock, Metal, Fusion, Funk, Jazz and so many others. It is easy to understand and makes sense, especially when you listen to the stylistically relevant examples used to explain principles.