In response to Matt's comment on reading (below), here is a brief annotated list of the most impactful books I've read.
Pride of place must go to John Piper's Desiring God: The Meditations of a Christian Hedonist. I've read it four or five times now and every time I read it, I am freshly challenged. It is, quite simply, the most riveting challenge to live a single-minded life of passion for God that I've read.
John MacArthur's Faith Works: The Gospel According to the Apostles was very helpful in clarifying my soteriology and pulling me out of some hyper-Calvinistic tendencies. Many other books could have done the same, but this just happened to be the one I read.
A Dissertation Concerning the End for Which God Created the Word by Jonathan Edwards, along with John Piper's commentary in God's Passion for His Glory was impactful in both my thinking and my grasp of the unifying theme of Scripture. I've gone on to read much of Jonathan Edwards and every book I read challenges me in both my thinking and my devotion to Christ.
C. S. Lewis's Surprised by Joy: The Shape of My Early Life, The Great Divorce, The Pilgrim's Regress, and Perelandra were all mind-blowing books for me. I continue to read and reread Lewis because he stirs my imagination like no other.
John Bunyan's The Pilgrim's Progress is a classic, whose images and metaphors have taught me much. J. I. Packer's A Quest for Godliness: The Puritan Vision of the Christian Life gave me a thirst for Puritan writings. John Owen's The Grace and Duty of Being Spiritually-Minded, The Mortification of Sin, and Thomas Brooks' An Ark for All God's Noah's are three of the best Puritan books I've read.
Martyn Lloyd-Jones' The Unsearchable Riches of Christ and Joy Unspeakable introduced me to new depths of spiritual communion with Christ. Thomas Schreiner's The Race Set Before Us: A Biblical Theology of Perseverance and Assurance clarified my understanding of perseverance and remains perhaps the finest piece of exegesis I've read.
The best book on preaching I've read is, hands down, Bryan Chapell's Christ-Centered Preaching: Redeeming the Expository Sermon. Chapell, along with Graeme Goldsworthy's books According to Plan and Preaching the Whole Bible as Christian Scripture (which I'm still working on) have helped me begin to understand the nuts and bolts of redemptive-historical hermeneutics and biblical theology.
Eugene Peterson's books for pastors, Under the Unpredictable Plant and Working the Angles have both ministered to me greatly this year.
Finally (making this a twenty-five item list, not five!) some of my favorite biographies are Iain Murray's two volumes on Martyn Lloyd-Jones, Arnold Dallimore's two volumes on George Whitefield, George Sayer's Jack: A Life of C. S. Lewis, and John Piper's The Legacy of Sovereign Joy.