After a period at Magdalen College, Oxford, he became the chair of Mediaeval and Renaissance Literature at Magdalene College, Cambridge. In spite of this position, he claimed that there was no such thing as an English renaissance. Much of his scholarly work concentrated on the later Middle Ages, especially its use of allegory. His The Allegory of Love (1936) helped reinvigorate the serious study of late medieval narratives like the Roman de la Rose. His late work, The Discarded Image, an introduction to Medieval and Renaissance Literature (1964), is an excellent summary of the medieval world view, the "discarded image" of the cosmos in his title.
In addition to his scholarly work he wrote a number of popular novels, including a popular series of fantasy novels for children entitled The Chronicles of Narnia; a trilogy of science fiction books: Out of the Silent Planet, Perelandra, and That Hideous Strength; and a novel based on Greek mythology Till We Have Faces.
The Chronicles of Narnia are by far the most popular of his works, and The Lion, the Witch and the Wardrobe, which is the first and most popular book in the series, has been adapted for both stage and screen. The Chronicles of Narnia borrow from Greek and Roman mythology, and traditional English and Irish Faerie Tales: Lewis cites George MacDonald as an influence (in The Great Divorce, the narrator is chaperoned in Heaven by MacDonald). However, the overall theme of each Narnia book is a Christian one. Likewise, Lewis's Space trilogy blends traditional science fiction elements with the exploration of the biblical themes sin, fall, and redemption.
Lewis's last novel was Till We Have Faces, and many would claim that it is his most mature work of fiction. It is a retelling of the myth of Cupid and Psyche. Again it touches on religious themes, but the connections with the specifics of his Christian beliefs are not as clearly delineated.
In addition to his career as an English Professor, and his novels, Lewis also wrote a number of books about Christianity, such as The Screwtape Letters -- letters of advice from an elderly demon to his nephew -- and perhaps more famously, Mere Christianity. As an adult convert he was very much interested in presenting a reasonable case for the truth of Christianity. Mere Christianity, The Problem of Pain, and Miracles (book) were all concerned, to one degree or another, with refuting popular objections to Christianity. He wrote an autobiography entitled Surprised by Joy, which describes his conversion (it was written before he met his wife, Joy Gresham).
Recently there has been some interest in biographical material concerning Lewis. This has resulted in several biographies, at least one play about his life, and a 1993 movie, titled Shadowlands after a story by Lewis. The movie fictionalizes his relationship with an American fan, Joy Gresham, whom he met and married in London, only to watch her die slowly of a terminal illness.
Many books have been inspired by Lewis, including the apologetics book A Severe Mercy by his correspondent Sheldon Vanauken, and numerous Narnia-inspired novels by various hands.
A claim by Lewis scholar Kathryn Lindskoog, that certain posthumously-published Lewis works are faked or misattributed, remains unresolved, despite claims that it has been debunked.