If you're already a theology buff and you haven't read all of these, put the ones you've missed on your reading list. You'll learn something in each one, I promise.
40 Questions About Interpreting the Bible by Robert L. Plummer is made up of short stand-alone chapters answering 40 common questions related to understanding the Bible. There are plenty of charts and lists to keep things uncomplicated, and enough humour and stories to keep things unstuffy. I don’t know of any introductory book on interpreting scripture that would be more useful for the lay person who desires to better understand the Bible than this one.
On God's Nature and His Work
Knowing God by J. I. Packer will help you understand who God is, what he has done for you, and cause you to love him more because of it. Years ago when I was writing posts on God's attributes, I referenced this book frequently because Packer has a way of expressing truths about God precisely. This is one of the most frequently recommended Christian books, and there's good reason for it. I know people who re-read this every couple of years, and there's good reason for that, too.
On the Trinity
Michael Reeves’ passion for the doctrine of the Trinity comes through on every page of Delighting in the Trinity: An Introduction to the Christian Faith. It’s obvious that for him the doctrine of the Trinity is not dry, irrelevant, or embarrassing, but the central truth of Christianity, “the truth that shapes and beautifies all others.” Reeves doesn’t assume that the reader has a background in Trinitarian theology, so this is an excellent choice for a student or new believer. And his passion for the subject makes it a good choice even for those who consider themselves well-studied in the faith. None of us are beyond more delight in the Trinity.
On the Work of Christ
Leon Morris wrote the definitive scholarly work on the cross of Christ, The Apostolic Preaching of the Cross. Happily for those of us who aren't exactly scholars, he took the material from his big book and made a version just for us, titled The Atonement: Its Meaning and Significance. Here Morris explains the terms associated with the atonement, like justification, sacrifice, and propitiation, so we can understand the meaning and significance of what Christ accomplished for us.
On the Big Picture
I have two recommendations in this category. The first is The World-Tilting Gospel by Dan Phillips. It's the whole-Bible gospel delivered to us in an energetic, easy-to-understand, earnest-but-never-preachy style. I suspect this book was written with newish believers and young Christians in mind—and it’s perfect for them—but it is also good for every believer as a reminder of the unabridged Gospel.
Second, there's D. A. Carsons' introduction to the Christian faith, The God Who Is There: Finding Your Place in God's Story. In fourteen chapters, Carson explains the big story of the Bible, the story of God's work in the world from creation through to final restoration. Out of the Ordinary's own Kim Shay wrote that this book assumes
the reader knows very little about the Bible, and would be an excellent tool for sharing with someone interested in Christianity, or even someone who has a lot of pre-conceived notions about it. Dr. Carson writes like a wise father figure, patiently, carefully explaining very difficult concepts in a way that leaves you thinking with delight at the end, "Hey, I understand that!"She's exactly right. If I could say it better, I wouldn't have quoted her!
Even if you consider yourself well versed in the big biblical picture, these two books will help prepare you to communicate the Christian faith.
Are there any easy-to-read doctrine books of classic quality that you would add to this list?