I have used many boxes of these books for discipleship over the last few years. None of these books are difficult to read. I have used them both with adults and high school students. You could read them in order, or you might want to start with Transforming Grace or Seven Reasons Why You Can Trust the Bible.
The Holiness of God, by R.C. Sproul
Outside of the Bible, this is one of the top three Christians books I have ever read. You will not see God the same way after reading this book. If you get the message of this book you will realize that everything is God-centered.
When I Don't Desire God, by John Piper
This is one of John Piper's best books for describing how God is glorified when we delight in Him as our highest treasure. This is also an incredibly practical book that helps us to learn how this is if hindered or helped.
Seven Reasons Why You Can Trust the Bible, by Erwin Lutzer
Treasuring Christ as Prophet means hearing the voice of Scripture as the voice of God. This is a very readable book that will help you have confidence in the Bible as God's own Word.
Transforming Grace, by Jerry Bridges
This book does an excellent job of clearly explaining that salvation is by grace alone and that the Christian life is based on grace as well.
The Joy of Fearing God, by Jerry Bridges
Along with Sproul's The Holiness of God, this is the best book I know of for helping believers stand in awe of God's majesty. This books is deep, and warm, and will cause you to bow the knee to God with reverence and joy.
Change in Thinking and Actions
How People Change, by Timothy S. Lane & Paul David Tripp
As Christians, we need to learn to be students of the human heart. This book does a very good job of showing how real heart-change takes place.
The Bible itself is the only book that is essential to read, but excellent non-inspired books can still be a major benefit to your Christian growth. There six books are great ones to start with for a solid, God-centered foundation.
Related: A God-Centered Philosophy of Discipleship