Left my job in 2014. Tried a few things that didn't succeed before selling programming ebooks came to my rescue (2018). From the danger of draining out my savings completely, I'm now earning more than double my needs (which isn't really that much for my modest lifestyle, living alone in a developing nation - $150/month).
What's more, my ebooks are all free to read online. Only PDF/EPUB are sold, and I've always given them away for free during launch week.
List of my ebooks: https://learnbyexample.github.io/books/ (topics include Regular Expressions, Linux CLI tools, Python and Vim)