Back in the early 2000s I quit my engineering job and went to music school for a few years. We used Mark Levine's excellent "Jazz Theory Book" to cover the theoretical aspects. It presents theory in the context of trying to understand how to improvise over jazz harmonies - I found it very useful. It's a textbook though: you're going to have to invest some time in it to get the most out of it.
Books aside, in my view the #1 thing you can do to help your music theory understanding is to train your ear: if you can't reliably identify all the intervals within an octave and identify major, minor, diminished and augmented triads, as well as the basic 4-note chords (major 7, minor 7, dominant 7, minor-7-flat-5) by ear, knowing a bunch of rules about tone-tone-semitone-tone-tone-tone-semitone won't be all that useful. Back in music school days I practised daily with EarMaster and within a couple of months had gotten solid enough at recognising intervals and chords by ear that it made all the other music learning I did subsequently much much easier. I am sure there are way better ear training tools now!
https://www.amazon.co.uk/Jazz-Theory-Book-Mark-Levine/dp/188...
Books aside, in my view the #1 thing you can do to help your music theory understanding is to train your ear: if you can't reliably identify all the intervals within an octave and identify major, minor, diminished and augmented triads, as well as the basic 4-note chords (major 7, minor 7, dominant 7, minor-7-flat-5) by ear, knowing a bunch of rules about tone-tone-semitone-tone-tone-tone-semitone won't be all that useful. Back in music school days I practised daily with EarMaster and within a couple of months had gotten solid enough at recognising intervals and chords by ear that it made all the other music learning I did subsequently much much easier. I am sure there are way better ear training tools now!