"Zero To One might be the first best-selling business book based on a Tumblr. Stanford student Blake Masters took Peter Thiel’s class on startups. He posted his notes on Tumblr after each lecture. They became a minor sensation. Thiel asked if he wanted to make them into a book together. He did."
That's an interesting origin story.
I was curious to see the notes in question. Found them here, for anyone else interested: http://blakemasters.com/peter-thiels-cs183-startup