David VanDrunen's book on the two kingdoms is finally out. Just got my copy. This is an important book--given all the angst and misinformation about "radical two kingdom theology."
Dr. VanDrunen discusses his book with Scott Clark here: Office Hours: Living in God's Two Kingdoms
Also, you might enjoy the recent discussion of the two kingdoms and the Belgic Confession, on Darryl Hart's blog (Hart on the Belgic Confession and the Two Kingdoms) with a follow-up by Wes Bredenhof (Wes Bredenhof on the Belgic Confession).