In my opinion, from the first list Steve posted, it would be most interesting to read a book by him on polarity. I'm sure he has lots of profound things to write about in fields like time management and business, but I feel that the area in which Steve would do the best job would be polarity. And, as Steve said, there is a much greater need for us right now to be concerned about ethical progress (being heart-centered).
Actually, more than polarity, I would really love to read a book on philosophy by Steve. One that presents his views on the various topics like skepticism, openness/willingness, and metaphysics in general. He has, of course, written much about this on his blog, but a more detailed exposition would definitely be wonderful. Steve could in fact even write the next book together with his wife.