I read many of his books when I was in my twenties and can recommend them (the last two are science fiction):
- The Screwtape Letters
- Surprised by Joy
- Miracles
- A Grief Observed
- The Problem of Pain
- Mere Christianity
- The Great Divorce
- The Four Loves
- Out of the Silent Planet
- Perelandra
He is a spiritual writer without necessarily identifying with any one religion, which I like. He chronicled his 'conversion' from atheism to Christianity in Surprised by Joy and Mere Christianity. Whatever I write about his books here cannot do them justice. Each book he wrote is its own treasure and there is much to discover in each of them. They will change your life it you let them.
I leave you with this quote that I found online today. Typical Lewis--he makes you think. Plato thought in much the same way--that all things exist as 'Forms' in an abstract state. In the case of human beings, they acquire a body at birth. I don't pretend to understand his philosophy, but I find it fascinating.