C.S. Lewis was a prolific writer. And profound. Although not a theologian, Lewis delved into spiritual matters in a practical yet penetrating manner.
You're probably aware of his famous apologia of the Christian faith titled, Mere Christianity, originally given as a series of radio lectures addressing the central issues of Christianity to a listening England during the uncertain days of World War II.
His children's literature is also popular. The Narnia series, of which The Lion, The Witch and the Wardrobe was put to film again recently (I quite enjoyed the 1988 edition) ignited renewed interest in Lewis the fiction writer.
I recently became aware of some "lost" stories of his. One is an unfinished novella titled, The Dark Tower. And it might never have been recovered but for Lewis's gardener, Fred Paxford, who saved a great quantity of notebooks and papers from the flames.
Evidently Major W.H. Lewis was not interested in any of the extra and sundry manuscripts still laying around the house after his brother's death in 1963; so he was going to burn them. Fortunately Paxford persuaded Major Lewis to put off the bonfire until a friend, Walter Hooper, could review the collection.
Hooper found The Dark Tower and another unfinished piece, After Ten Years. These stories, along with some other published fiction, are published as a collection which I'm reading right now. A nice addition that finally rounds out all of Lewis's fiction: Narnia, Screwtape Letters, and his science fiction trilogy. Highly recommended.