Simply the best for people in your situation is Edly's Music Theory for Practical People. I love this book, and wish it had been around when I was teaching full-time. It's a clearly written and logical development of theory without sounding academic or stuffy. It's full of diagrams similar to what I used to use in the '80s (except his are legible and mine often weren't). I love it because
A. It's built in a logical order that allows you to actually LEARN theory as music
B. It doesn't assume that the only music that exists is either Western classical music or jazz. He uses examples from all over the map (and good bit of the world) to demonstrate.
C. It doesn't assume you know how to play keyboards
D. It doesn't limit itself to treble-clef instruments, and discusses bass function
E. It doesn't assume you know how to read music, but he also makes a good case for learning the basics of the language of music.
F. It's written in a light-hearted and humorous manner without being silly, childish, or lame.
HIGHLY RECOMMENDED!!!
Play, Learn, Music, theory, instruction, books, piano
John