C.S, Lewis

Versatile
Thought-provoking
Genius

I used to sit in his favorite reading chair in the chaplains office at Cambridge.

The Narnia books were probably my favorite books growing up. The only other thing I have read of his was The Screwtape Letters. It was interesting.

The Narnia books are obviously meant for children, but there are lessons for adults as well. Obviously, there are strong Christian themes throughout, but I think that the narratives stand up on their own for non-religious readers.

On the other hand, the Space Trilogy (Out of the Silent Planet, Perelandra, and That Hideous Strength are meant for adults and explore deeper philosophical and religious concepts. Perelandra is one of my very favorite novels, and That Hideous Strength seems to apply to our modern day even more than when it was written.

Mere Christianity is possibly the most popular book ever written as a persuasive defense of the Christian faith. Most of the arguments that Lewis makes in it are not necessarily new to him, but he makes them in newly interesting ways.

The Screwtape Letters is a "story" told from the perspective of a senior demon instructing his young nephew in the best tactics for tempting humans. I personally put this in the category of "Christian Living" books -- very insightful. I also really enjoyed the stage adaptation, which is close to finishing another tour.

The Great Divorce is a story about the separation between heaven and hell and attempts to establish that people in hell might not be happy in heaven. It's probably been almost 20 years since I read it, though.

That's all of his stuff that I've read, but The Abolition of Man, The Problem of Pain, and A Grief Observed are also all very popular (the last of these is the basis of the movie Shadowlands).

Read all of his books, then start over and read them again.

Mere Christianity changed the way I look at the world and more importantly, the people around me

Lewis was a brilliant writer. I don't think anyone since Lewis has been better at explaining the Christian faith.
There are a series of essays Lewis wrote that are available as audio on YouTube. Just search for "CS Lewis essays". Each essay is approximately 10-40 minutes long. Reading (listening) to these essays will help you realize just how insightful Lewis was able to think.