If I ever get around to writing a handbook on preaching, one of my primary road rules will be, "Read good theology, lots of it!" I quite often find it to be the case that when I try and read practical resources to prepare for preaching (commentaries on the biblical text from the lectionary, stuff off textweek, etc.), the pay off is slim. I don't mean to imply that these resources are somehow poorly written or un-interesting in their own right. Clearly, there's a direct application and connection- it just doesn't, at least in my method of sermon preparation, prepare in a fruitful way for the task of preaching.
On the other hand, when I stay engaged with solid theological resources, especially books that aren't obviously addressing a theme or text for the Sunday sermon, they do in fact in surprising ways prepare for preaching.
I consider this one of God's serendipitous benefactions. Read theology for its own sake, the joy of it, and the wonder, and it will prepare you to preach the Word of God come Sunday. Read widely, far away from anything that seems like a sermon handbook. Read difficult stuff. Explore the wonder of it. And you will be fed.
Two books I'm reading right now are stellar examples of this. James Alison's new collection of essays, Undergoing God: dispatches and scenes of a break-in, is a wonder. You can check out most of the essays at his web site, www.jamesalison.co.uk
The other book, also a collection of essays, is by the late and great Colin E. Gunton, Father, Son, and Holy Spirit: Toward a Fully Trinitarian Theology . For example, some aspect of the following will find its way into my Easter sermon: "In sum, the lesson we can learn is this: if you want to understand how God works in our world, then you must go through the route God himself has given us--the incarnation of the eternal Son and the life-giving action of the Spirit. Let me repeat: the Trinity is about life. Irenaus is the writer of that great sentence, often heard from him: the glory of God is a human being truly alive." (11)
It is periodically the case that resources on-line or in books of practical theology bear such proclamatory fruit, but each time I set aside time to read great theologians, I come away once again with a sense of the wonder that is this holy conversation, and the over-flowingness of such discourse for the life of faith and preaching.