I've written a couple of books that sell on Amazon and Apple iBooks - it takes zero minutes per month work for me, but brings a nice little side income. I won't get rich, but it's nice to have the money come in for doing nothing after I hit the "publish" button. Even the printed books cost nothing. I'm about to hit publish on a photo coffee-table book from my three years around Africa, and I'll write an Africa guide…
Would love to help in any way we can on your next book (I'm the founder of Reedsy, https://reedsy.com)