Check out sermons here:
http://www.redeemer3.com/store/index.cfm?fuseaction=category.display&category_ID=11
I listened to "Blessed are the poor" on my last Austin-Henderson commute. Keller speaks on what is, for me, one of the most difficult passages to square with pervasive Americanized Christianity. I have also listened to 'How to Change" - great discussion on the fruit of the Spirit.
Keller is sober and intellectual. I believe he speaks at a Presbyterian church in New York. I hear he has written only one book. He seems like the type who would write more...
Evan