Another area of study that I feel is often neglected is composition. Granted it is a difficult thing to nail down because there is a fair amount of opinion and personal preference involved. One of the ways I like to study composition is a master study or ten. The thing to remember about master studies (especially in regards to composition) is that they do not have to be detailed, rendered, longterm exercises. Sometimes you can get more out of simple three values studies in your sketchbook. In essence you are reverse engineering thumbnail sketches.
I have been interested recently in trying to figure out the graphic appeal of James Bama's work, in particular his series of Doc Savage covers.
There are others, but these are a great starting point. Plus they are all available and affordable.