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.
I will put The Great Divorce on the list. I might reread Mere Christianity first; it’s probably been 25 years. Lewis was a great writer and thinker.