The first compilation of some of Jack's (as he was referred to by friends and family) correspondence, was, Letters of C.S. Lewis. This was followed by Letters to an American Lady, C.S. Lewis' Letters to Children, The Latin Letters of C.S. Lewis, and eventually, three large volumes of The Collected Letters of C.S. Lewis. I have not read all of these, but I have read Letters of C.S. Lewis, his Letters to Children, and some of the Collected Letters. These add a whole different dimension to understanding his thoughts and development as a person and of his faith.
Many other writers, fascinated by this man have written biographies or analysis of his works. Some that I can recommend, beginning with the biographies, are: Tolkien and C.S. Lewis: The Gift of Friendship, Remembering C.S. Lewis, C.S. Lewis at the Breakfast Table, and Jack: A Life of C.S. Lewis. For analysis: C.S. Lewis as Philosopher, Planet Narnia, C.S. Lewis Companion and Guide, and C.S. Lewis for the Third Millennium. This is by no means an exhaustive list, but one from which I can personally endorse. They all have their points of view, some of the writers are Roman Catholic and some are Protestant which at times comes through, but all are beneficial.
There have also been documentaries which include: The Magic Never Ends, The Life of C.S. Lewis, and The Life of C.S. Lewis: Through Joy and Beyond.
For a complete bibliography of his writings, see here.