Publish books on Amazon. I've published 4 and make about $3k in profit per month, after paying for ads on amazon.
Don't write them yourself, that's a lot of work.
Instead, do key word research about what books sell in non-fiction categories (fiction is a whole different ballgame). Look for keywords that 1) auto complete in the amazon search bar, and 2) have less than 4,000 matching books, and 3) have an average Amazon BSR of less than 150k.
The above isn't easy, nor is it particularly difficult.
Once you've found a good keyword, create an outline on the subject. You'll have to do a bit of research here, but this can be done in a few hours to a few days. The more time you spend here, the better your result will be.
Then, hire a ghostwriter to produce a 30k-ish word long book. This will run you around $1000, and produce a book long enough to turn into a 3+ hour audiobook on Audible (audiobooks over 3hrs get significantly better royalties than less than 3 hours).
Get a cover made. This will run from $50ish on fiverr to several hundred on 99designs or upwork. Don't cheap out, good covers are important.
Publish on amazon. Run ads.
There's obviously more too it than this. If interested, look up the Mikkelsen twins or Dane McBeth on youtube.