If you only read one CS Lewis non fiction make it mere Christianity. I don't agree with everything he says but I like how he explains it and he keeps it simple, which is good for anyone sometimes. I read all the Narnia books when I was young and have tried to read some of his other non fiction but could not get through them. Well I read Screwtape letters. I tried The Four Loves and simply the language and style kept me from enjoying it.