Art Katz mentions this book quite frequently in a lot of his speakings, I received it in the mail this week and read it in two sittings, it is only 70 pages long, and I recommend it.
It is Christ and the Powers, by Hendrik Berkhoff.
Berkhoff does an analysis of most, if not all, of the scriptures where Paul mentions the "Powers," "Principalities," and the things of the air which we take for granted. It was written during a theological time period where it was common for theologians to assume that these things were just an ancient manner of speaking, and many of the theologians were actually rather embarrassed by what Paul had said.
Berkhoff does a decent job explaining what these things were, and opens up a great theme for discussion. It seems to me that this is a very pertinent and relevant discussion considering the dynamo of current events and how they are being shaped in our day.
Hope it blesses, and inspires you to be the "Church" as it ought to be, and not as it is presumed. This was the most dynamic aspect of the book. It filled me with hope and a new found respect for just what it was my Lord did on Calvary.
Praise Him
-Jeremiah