Another book I read recently is called Glimpses of the Devil, by M. Scott Peck. Although there was not a lot of study material per se, it was a very interesting account of two supposed cases of demon possession. The purpose of the book is to convince the reader of at least the possibility of such supernatural concepts — demons, possession, the existence of the devil.
The accounts are quite gripping, especially early on in the book — I could hardly put the book down — but in the end I found myself less than thoroughly convinced.
The book brought to mind C.S. Lewis' concept of good and evil (bear in mind that I am paraphrasing; it has been a while since I read Lewis' works). He argues that there is no such thing as "evil" in and of itself — evil is simply a spoiling or negating of that which is good. It is similar to the concept of heat and cold; technically, there is no such thing as "cold", it is only the absence of heat.
I remember my grade-school teacher trying to explain why vacuum cleaner doesn't "suck," and getting very confused by her explanation. Now I realize that in the same way, a "vacuum" occupying an amount of volume is not an actual "thing," it is the absence of a "thing."
These could all be semantics, or a shifting of your reference point. I think the concept of the devil as a thinking, active being is difficult for me to accept. All personality is a facet of the goodness of God; if Satan is totally devoid of goodness, does it have a personality?