C.S. Lewis is arguably one of the greatest Christian thinkers and authors of all time, and his impact on our modern understanding of Christianity has helped many grow closer to the Lord.
1) Chronicles of Narnia
The Chronicles of Narnia is Lewis' epic and life's work much in the same vein of J.R.R. Tolkien's Lord of the Rings. The imagery of Christ through Aslan the Lion is quite evident along with the crucifixion type scene in the Lion, the Witch, and the Wardrobe.
2) Mere Christianity
Mere Christianity Defends and explains the core of Christianity and allows readers to experience and really know the impact of the resurrection of Jesus, while proving He is who He says He is.
3) The Screwtape Letters
C.S. Lewis creates a masterpiece in telling the story of a high in command officer for the Devil following a man who becomes a Christian. The demon throughout the book chronicles how he tempts and tries to persuade this man into doing evil. It's a great look into the otherside.
4) The Pilgrim's Regress
In this book Lewis revises the classic Pilgrim's Progress story and updates it with the politics, philosophies, and point of view of the 20th Century.
5) The Great Divorce
The book covers the Christian conceptions of Heaven and Hell, and creates a narrative in the fashion of Dante's Inferno. The premise focuses on the unknown of the afterlife.