Clive Staples Lewis used the initials “C.S.” as his author name.
The initials represent his first and middle names, Clive Staples. He generally published his books as C.S. Lewis, a form that became one of the most recognizable author signatures of the twentieth century.
Lewis’s familiar nickname, “Jack,” came from an experience in childhood involving a dog named Jacksie. Although friends and family used that name, his books were published under C.S. Lewis rather than Jack Lewis.
The additional initial in “C.S.L.” is not part of his standard author name. His surname was Lewis, so “C.S.” alone identifies the two given names used in the pen name.