Here are some of the best books (currently available for purchase) on helping with sermon preparation and on the art of preaching.
Alistair Begg, Preaching for God’s Glory
Bryan Chapell, Christ-Centered Preaching
H.B. Charles Jr, On Preaching
Bryan Chapell, Gospel-Centered Sermons
Edmund Clowney, Preaching Christ in all of Scripture
Mark Dever and Greg Gilbert, Preach: Theology Meets Practice
Graeme Goldsworthy, Preaching the Whole Bible as Christian Scripture
Sidney Greidanus, The Modern Preacher and the Ancient Text
David Murray, How Sermons Work
Bryan Chapell, The Hardest Sermons You'll Ever Have To Preach
Steve Lawson, The Kind of Preaching God Blesses
Ligonier Ministries, Feed My Sheep: A Passionate Plea for Preaching
Martyn Lloyd Jones, Preaching and Preachers
John MacArthur, Rediscovering Expository Preaching
Alec Motyer, Preaching?
John Piper, The Supremacy of God in Preaching
Haddon Robinson, Expository Preaching
John Stott, Between Two Worlds
Jerry Vines, A Practical Guide to Sermon Preparation
Warren Wiersbe, Preaching and Teaching with Imagination
Would you add any books to this list? Which of these is your favorite?