Self Publishing is mainstream today. When you purchase an eBook off of Amazon there’s a pretty good chance you’re buying a self-published book. Self-publishing is also ridiculously easy. I tried this a few years ago and couldn’t believe how simple the process was. To self-publish a book you’ll first need to write and edit it, create a cover, and then upload to a program such as Amazon’s Kindle Direct Publishing. Don’t expect instant success though. There will need to be a lot of upfront marketing before you can turn this into a passive income stream.
Also, I know from my personal experience that when you only do something because you think it’s a money-spinner you end up nearly killing yourself to get the idea over the line. I remember creating a careers advice site that bored me to tears. Every word I wrote, hurt a little. All 200,000+ words! Do not put yourself through this pain. I know I won’t ever again.