I made two TypeScript videos for Pluralsight ("ES6 with TypeScript" and "Practical TypeScript Migration"). I put them together in the evenings and weekends using the knowledge I got at my day job and from working on TypeScript open source projects like grunt-ts (and a lot of research).
It was a lot of work, but it's a great passive income now that they're done, and I'm quite proud of how they turned out. The Pluralsight authors are a great professional network to be plugged-in with, and being an author is a pretty unique differentiator on your resume.
I'm going to put together a third course soon.
They're always looking for new authors. https://www.pluralsight.com/teach