I teach a three hour workshop called Music Theory for Bluegrass which is most of the theory a person needs to know for bluegrass and nothing more. When I studied theory in college, I found most of what I learned didn't apply to the bluegrass world since it was based on classical music. It was a very inefficient way for me to learn what I wanted to know, so I ended up taking private lessons in music theory. When I put this class together, I was careful to only include things that applied directly to bluegrass.
Here is the handout from the class: Music Theory Handout
Here is an interesting look at the physics of pitch and western scales.
Physics of Pitch