Clive Staples Lewis commonly used the nickname Jack throughout his life.
Lewis adopted the name Jack during childhood after a beloved dog named Jacksie was killed by a car. He initially insisted that family and friends call him Jacksie, which was later shortened to Jack. The nickname remained his preferred form of address into adulthood.
Although his books appeared under the name C.S. Lewis, people who knew him personally generally called him Jack. The name is especially common in biographies, letters, and recollections by his friends and colleagues.
The nickname is sometimes mistaken for a pen name, but it was not a literary pseudonym. C.S. Lewis was the authorial name used on his publications, while Jack was the personal name used in ordinary life. His full legal name was Clive Staples Lewis.