Who is C.S. Lewis from A Severe Mercy and what is their importance?

C.S. Lewis, or Jack, begins his presence in the story as the author of some of the most thought-provoking of the books Van and Davy read in their initial investigation into the Christian faith. He evolves into a close friend, advisor, and confidante as he and Van exchange letter after letter, and meet for long conversations about the Christian life and all of its challenges. Jack and Van become even more closely bonded when Davy dies and Jack anticipates and then experiences the passing of his own wife. His letters are the source for the title of this book, and much of the wisdom and perspective Van gains through the years it chronicles.