Just finished. It is a good provocative read. It is written in a nice narratival theological style, which definitely works within the spirit and ambit of the Barth 
style (i.e. engagement with Holy Scripture throughout). Philip re-places a doctrine of the devil into the second article, so, Christology and Soteriology (think Apostle’s Creed) versus the traditional placement as found in the first article with reference to original creation and God’s providence. This reorients thinking the devil from emphasizing him as a fallen angel, and instead sees him primarily in the scenery of the wilderness, as the adversary (of Christ), as adventitious, and anarchic. These three themes serve to organize the way that Ziegler seeks to place the devil within a dogmatic location of the second article. And it shouldn’t be forgotten that Ziegler is doing all of this work as a type of revitalization of a satanology for and within Reformed theology, proper.
One question I had going into this book was to see if Philip would hold to the idea that the devil is “personal,” or if the devil has an ontology of diabolical sorts. In the long and the short of it: yes, Ziegler affirms a type of nothingness beingness (my phraseology) which pairs well with Barth’s doctrine of sin or “nothingness” (das Nichtige)—which you can read about in CD III/3 (I also have some passages on that in my Barth Reader).
As a North American evangelical, like myself, I am conditioned to think of the Devil in the first article. As someone who reads Barth and Torrance and Calvin a lot, I am re-conditioned to think within the second article. And so, this clash continues for me, particularly when it comes to such matters as a satanology represents. Beyond that, as a basic evangelical Christian, there are too many real-life encounters that I have had with the demonic—ones that mirror those in the Gospels etc.—that experientially keep me from falling into any type of crass physicalism when it comes to the dark spiritual realm. Even so, the themes that Ziegler develops, therein, don’t contradict that reality, it just textualizes them in ways that some readers might think envelopes this matter into a purely intellectual exercise; but that would be wrong. Take it up and read for yourself.