This book was a real challenge for me, so my rating may not be entirely fair. The author seemed to take for granted the background knowledge of the reader, so I felt like a lot of material was glanced over, and that conclusions he made were not well supported. I felt like he was saying, "This is the conclusion, and I don't need to back it up because it's so obvious."
The chapter on the Persians, and the later chapters on Judaism, Christianity and Islam were the strongest. The other chapters were intended to show that there is not a universal idea of a single, evil adversary. I think the author should have spent more time on those strong areas and peppered the rest throughout as a means of contrast.
In spite of that, I think the author (in those stronger chapters) does make a very good argument that the idea of an evil adversary was created and exploited out of political need.
Finally, I felt the last chapter was extremely dated material. If a later edition is ever published, the final word needs to be re-written.