Platonic Prince

I am concerned with characterizing the Devil in literature. Right now I am playing with: 1) a definition of the Devil and  2) that which is common to ALL of his manifestations. The books I have selected so far were not difficult to come by (considering that I own them all), and seem--each and every one of them--critical to characterizing the dark prince. Considering the immensity of my task, this list is sure to grow--exponentially. I want to KNOW his essential nature, the form behind the form, if you will.