Simply put, this work is about knowing God. He discusses the why and how of knowing God, and the differences between “knowing about God” and actually “knowing God.” Packer clearly portrays God in all his complexities. So many times, God is simplified and smoothed over, to make him appear more palatable, as more of a cosmic force for good rather than an actual “being.” Knowing God explains God's attributes not merely for theological study, but for their practical implications on our faith, lives, and worship.
So often when we talk about God, it is about what he wants from us, how do we please him, is this action sinful or not; we are actually focusing more on ourselves. When we do talk about him it is usually vague and speculative, much like we talk about aliens or Bigfoot. One of the first points Packer brings up is that we can know God, not because we can figure him out, but because he has revealed himself to us; through scripture, His Son, and His Spirit. Since he has manifested himself to us, it should be every Christian primary duty, no desire, to learn all we can of our great God.
Again I can't recommend this book enough, for new and mature Christian alike.
One final note, chapter 19, “Sons of God,” Packer's treatment of our adoption by God into his family, is worth the price of the book. I had never before considered the fullness of this doctrine.