The author offers a detailed look at the gradual changes in ancient Jewish and Christian thought regarding the identity of ha satan, the "satan" figure we now typically think of as God's foremost adversary.
It becomes clear that early on in the OT, both ha satan and its verbal form "to oppose or kill" sometimes refer to human agents (1 Sam 29:4), other times to the angel of Yahweh (Num 22), while other times to a member of God's heavenly court (Job). Only in later Christian tradition does the idea of Satan as God's arch enemy emerge.
The author also has a facinating section on the intertestamental literature's varying views on demons, evil, and election.
Since the author's aim was fairly narrow, he never touches on the implications for how his research might reorient our view of Scripture.
However, it seems to me that if we accept his conclusions, it makes the relationship of God and evil much more messy than I have often heard it portrayed.