I've recently stumbled upon this infographic showing a brief history of content marketing, compiled by Joe Pulizzi, founder of the Content Marketing Institute (CMI). The infographic was updated in 2016, so it's already a year old since. But I was intrigued by it especially the offline history pre-2000.
CMI is the leading content marketing education and training organisation. The site is packed full of useful articles - if you've never visited before, I suggest that you start from "How To Guides" from the right-hand side menu.