Hi, Trev.
If you're at the stage where you want to understand a little more about what you're playing (theory) and how to find it all on the fretboard (the whole fretboard), then I know you'd benefit from reading
PlaneTalk. There's nothing overly complicated about the basic structure of music, and it's certainly helpful when writing songs, something I know you're into. PlaneTalk would have to be one of the easiest approaches to a complicated subject, and I can say with confidence that the lesson it teaches will stay with you forever more. You'll never look down at the fretboard the same way again, once you digest what it's all about.