Densetzu said it well, regarding Lewis and his use of mythology.
My favorite is the Great Divorce. I think Lewis has a solid grasp on human nature, which I really appreciate.
I also think that the Chronicles of Narnia give glimpses of many different spiritual principles, i.e. spiritual blindness(the last battle), elements of addiction and bondage,( the silver chair) how God works differently in different lives,(the horse and his boy) and of course redemption (lion witch and the wardrobe) etc.
Right now I'm reading through the Problem of Pain. If anyone else has read it, it would be fun to discuss. Also, I had a Chesterton phase and I love the Father Brown mysteries. : )