C.S. Lewis builds Christian doctrine from the ground up, for both believers and non-believers alike. He does not assume that a reader already knows x or believes y. He expertly (and extensively) uses metaphor to help mere humans understand the incomprehensible facts about God.
For the non-believer, it is a great opportunity to get a bare-bones explanation of everything Christians believe. For the believer, it is a great help in understanding why you believe what you believe. Many Christians, without really realizing it, believe things without knowing why. Lewis's explanation of Christian doctrine from the ground up helps the Christian fill in the blank parts of their theological understanding.
There is not one single day in which an idea from this book doesn't pass through my mind, and I use logic from the book on a regular basis to explain what I believe and why. I would highly recommend this book.