PBS American Masters program “Rod Serling: Submitted for your Approval.” It is a quality piece and quite comprehensive. It is available on VHS and DVD.
If you prefer reading, the two biographies I am familiar with are both unauthorized and although comprehensive, they take liberties. They are: “Serling: The Rise and Twilight of Television’s Last Angry Man” by Gordon F. Sander, 1992; and “Rod Serling: The Dreams and Nightmares of Life in the Twilight Zone” by Joel Engel, 1989.