Creating books that guide people to do X and Y in a niche is the real money maker. If you're an expert in some niche and publish a guide book, you can earn from royalties or expand the content with later editions. Here's what I've forgotten I learned while researching about passive income.
Trading guru's don't necessarily have to make a lot of money trading. They just need to convince you they know how to trade and make money to sell you a course. And people buy this promise thinking about the results but matters little if you succeed at replicating those results, you already paid for the course and that's enough. If you do win, your testimony becomes a bonus for their self marketing campaign.
If you publish a guide book, you can earn from royalties, get what you wrote cited, and even if it gets pirated, there's always someone out there that's willing to pay for the premium. You initial capital can just be time and effort to make one. Selling hard copies can be costly. They take time to produce and deliver. Digital copies are faster to share, less cost to "make", and payments can be instant.
There's no need to reinvent the wheel. There's going to be someone asking how to do de-stress, garden, or cook out there and it doesn't have to be author X to give them the answer. Sure there's a lot of competition, but there's still a market for this because there's a demand per generation.
A lot of the self help book genre produces ideas that sound so profound until you read plenty of them and realize it's all repackaged from some ancient proverbial advice. The time's have changed but human nature didn't get the memo so each generation gets stuck with a history that rhymes.
Someone else made a better guide? no problem. There's still going to be people that would be interested buying your guide as long as you know how to market yourself in your niche.
Wouldn't AI just produce the guide on a prompt? that's a fair take. But not everyone would prefer prompting their guides, have access to internet/electricity, or sure about their AI not hallucinating facts.
If you're asking AI for guides, there's a good chance that it may insert information that isn't accurate and you wouldn't even know about it because you're not an expert on the subject your asking a guide about.
I had a brain blast thinking about this stuff in less than 15 minutes. Sometimes ideas just flash so I write them before they're gone.
Thanks for your time.