I had read Chronicles of Narnia as a child, and Mere Christianity in my late 20’s, but I only discovered this gem about 10 years ago. I had a long drive ahead of me, and I downloaded the recordings for the trip. I had run across it by accident while looking for something else. They’re are now available on YouTube. I was amazed at how timeless it is. Cleese does a fine job of delivery, too.
Mere Christianity was my second. It’s my personal favorite book outside the Bible.
The Great Divorce is also a must read. It’s kinda like science fiction and he actually said he got one of the ideas out of a story in a science fiction magazine (a time traveler that could go back in time but not alter anything to the point that raindrops would fall through his body like bullets.