Lewis is one of the most well-known Christian voices. His Chronicles of Narnia are classics now and Mere Christianity is read the world over. His spiritual autobiography Surprised by Joy and Reflections on the Psalms are also wonderful. For me, A Grief Observed, his reflections after the death of his wife, is the most powerful of Lewis' writing.

C.S. Lewis.org
Into the Wardrobe: The C. S. Lewis Web Site