as far as theory shmeory is concerned, that E7 can be called the secondary domiant V7/ii, where ii is the A minor that immediately follows. many times it is a secondary dominant when a major or dominant 7 chord occurs where a minor would be expected
I tried to read what it said in wikipedia, I guess theory is not my strong point(what an understatement), it just looks like you can intellectualize any note to be in a # of scales or modes at the same time depending on various factors(key).
I say the blueprints have all been layed out by Jerry, listen to the big picture, not necessaraly (sic)note for note but what techniques, up to down, down to up in melody, bends, dynamics, and by all means put that book down and pick up a guitar, it will all fit together, There are only 12 notes...in western music, that narrows it down a bit. I think in Dupree's you can play them all but one or two, Oh and by the way do you like the older versions, or the newer 70's and 80's with the envelope filter?