C.S. Lewis commonly used the nickname Jack.
Lewis adopted the name after the death of a childhood dog called Jacksie. He disliked being called Clive and eventually asked family and friends to call him Jack instead. The nickname remained with him throughout his adult life.
Many biographies, letters, and recollections therefore refer to “Jack Lewis,” even though his published books appeared under C.S. Lewis. “Jacksie” was the earlier childhood form connected with the dog, not the name most commonly used by his adult friends.
The nickname also appears in the name of the Oxford literary circle known as the Inklings, whose members included Lewis and J.R.R. Tolkien.