https://lgkmarketingcc.com/content-king-said-better/
The book seems to be J. W. Click, Russell N. Baird, Magazine Editing and Production. W. C. Brown Company, 1974, 274 pages
https://www.worldcat.org/title/magazine-editing-and-producti...
https://books.google.com/books?id=lMpHwLnvsvAC&q="content+is...
Earlier appearances in the 1960s refer to educational films:
https://www.worldcat.org/title/toward-improved-learning-a-co...
https://books.google.com/books?id=LiQgAQAAMAAJ&q="content+is...
Aguments over provenance notwithstanding, my view is that the aphorism is a convenient bit of stage distraction attractive to media monopolists themselves aware of the real truth: network control is emperor.