A number of years ago I stumbled upon Thomas Weinandy's book, Does God Suffer (Notre Dame, 2000).  To my mind it is the best thing available on the impassibility of God.  In particular, I have re-read chapter 8, "The Incarnation--The Impassible Suffers", numerous times.  Reading this chapter alone is like taking a first-rate seminar on Christology.  If you can find a copy, this book is highly recommended.  Here is an essay by Weinandy that appeared in First Things magazine back in 2001, summarizing the heart of his argument.