In the plethora of books C.S. Lewis wrote, there is not only a diversity in topics and themes, but a unity woven throughout. All his writing incorporates an underlying emphasis of Christianity (in the allegorical form throughout the Narnia series, and much more bluntly in non-fiction works like Mere Christianity). Literature ranging from theological non-fiction to heroic fantasy is amassed in his resume, exemplifying the range of Lewis's imagination. Alan Jacobs, author of The Narnian (available here), states in his book that:
"Lewis's mind was above all characterized by a willingness to be enchanted and that it was this openness to enchantment that held together the various strands of his life - his delight in laughter, his willingness to accept a world made by a good and loving God, and (in some ways above all) his willingness to submit to the charms of a wonderful story, whether written by an Italian poet of the sixteenth century, by Beatrix Potter, or by himself. What is 'secretly present in what he said about anything' is an openness to delight, to the sense that there's more to the world than meets the jaundiced eye, to the possibility that anything could happen to someone who is ready to meet that anything. For someone with eyes to see and the courage to explore, even an old wardrobe full of musty coats could be the doorway into another world. It is the sort of lesson a child might learn - even a stubborn, independent child - if his mother has died and his father and brother are often away and he spends his days alone in an old house full of books, thinking and drawing and writing and thinking some more." (Jacobs, "Intro" The Narnian, p. xxi)
It is evident while reading any of Lewis's writing that he had an incredible mind and imagination. The interweaving of Christian allegory in the Narnia books is no easy feat and he does it seemingly with incredible ease. After all, the seven books in the Narnia series were written within six years of each other, leading me to believe that the stories encased in his creative mind were just waiting to be put to paper.