I can think of no other author besides John Piper to write on the supremacy of God in preaching. In this book he challenges preachers to keep the "main thing the main thing" in their preaching and teaching ministry. Great resource.
What did Jesus really mean when he told Nicodemus we needed to be "born again"? John Piper explores the answer to that question in this book. I really enjoyed reading this and use it in my sermon preparation from time to time.