Learn Basic Piano Skills
Strange as it may sounds, I don't think I really understood the guitar until I started playing a bit of piano. Really helped put harmony into perspective and a lot of things just seemed to make more sense.
I'd highly recommend buying a basic keyboard (you can get reasonable sounding small keyboards very cheaply!) and just having a play around!
Some of the lessons I found that I thought explained stuff pretty well, though all have things I like and dislike.
This one has a classical approach that would suit someone playing a real piano or weighted keyboard.
I can't find the list of links I made for this so I'm going to have to come back and add these shortly!