A strange book; I'm not entirely sure what to make of it. Kelly's premise, and he is quite serious, is that Satan is a badly misunderstood fellow. That the Satan we know today bears little or no resemblance to the Biblical Satan.
As far as I can tell, Kelly is a believer in the Bible and in Satan, and wants to set the record straight. He doesn't appreciate the legends that have sprung up about Satan since the writing of the Bible. Satan, for example, has nothing to do with the serpent in the Garden of Eden; that association began with the early church fathers. He has not yet been thrown out of heaven. He is not evil, and certainly will not be put in charge of punishing the damned in Hell.
Rather, Satan is merely an Accuser, and a useful one at that. Kelly begins with the oldest Biblical books, travels through the Septuagint translation and inter-testamental writings such as 1 Enoch and Jubilees, and wraps up the first half of his book with the Dead Sea Scrolls and the New Testament. In these writings, we see the picture of Satan remaining largely unchanged: Satan is a functionary of the Divine Government, charged with testing and disciplining mankind. He is suspicious of everyone, including Jesus. He is hostile to Jesus' followers, constantly trying to trip them up and then lodge complaints against them before God. He is God's Chief Tester, at worst a homicidal liar.
Then in the second half, Kelly methodically traverses the next 2,000 years and how Satan's name has been corrupted.
I agree with Kelly that the understanding of Satan/Devil/Lucifer has evolved quite radically. I'm just not sure I agree with his timing. For example, I think Satan had evolved into God's adversary in the minds of most believers before the New Testament was written, and I think the same about Satan's connection with the serpent of Eden. Moreover, different Biblical writers appear to have had different ideas. I just don't think the line is as crisp as Kelly draws. Nevertheless, it's a book to make you think, and though the writing is a bit dry, I did enjoy the discussion, so I'll give it four stars.