Might be more accurate to say that he's the grandfather of modern business productivity/sales advice works. I'd say that the secret grandfathers of modern self-help are Jose Silva and his Silva Mind Control, along with Werner Erhard of est and Hubbard's Scientology. It was of course highly influenced by that sort of productivity advice through Holiday Magic/Leadership Dynamics, but was also very influenced by Theosophy through a science fiction lens that turned everything once supernatural psychic.

That's how we end up with all of these mind-training, success-mindset works that make up most of self-help now. It's a weird culty stew.

edit: Mill is a more noble origin for modern self-help than it deserves. His descendant is How to Win Friends and Influence People which is altogether more wholesome, more akin to Scouting (note Carnegie's constant advocacy of Toastmasters.) Modern self-help is trying to convince you that you can hypnotize people, memorize books, learn languages in a month, build billion dollar businesses, and bend probability through sheer concentration and adherence to miracle systems and new psychologies explained in listicles.