Seems like some in the comments are having a hard time calling the author's income "passive". Just an alternative - if you can find a profitable niche, you can

Seems like some in the comments are having a hard time calling the author's income "passive". Just an alternative - if you can find a profitable niche, you can have the book ghostwritten. A non-fiction book of around 30k words (~110 - 180 pages, depending on trim size) can be produced for $1000 (or less, if you can find a diamond in the rough on upwork). The cover can run anywhere from $100 - $800 depending on the quality you want. So, from $1000 - $2000 you can basically have a book written for you. Choose a pen name and sell it on Amazon.