Every category that can be imagined, as far as I can tell.
What non-writers who want to make money usually do is try to use AI or some automated or semi-automated process to turn "something" into a book. Think low-content or niche stuff like day planners, coloring books, puzzle books, word search, cryptogram, cross words, etc.
While it may sound funny, Amazon's reach is so massive that you'd be surprised how many people on a given week searched for the phrase "rugby puzzle book" - just an activity book themed around Rugby. Maybe as a gift for someone, who knows.
Of course, you can use AI to help you write a novel, too. Steamy romances and erotica are the best Amazon sellers, sadly to the reputation of the culture, but it is what it is. So far I have about 65 'books' ranging from kids coloring books, puzzle books, short children's stories, etc.
It's hard to make any money due to the saturation of the market as KDP has gotten so huge by this point, you have to run Amazon ads to make sales, and figuring in the cost per click with a $2-$3 royalty per sale, you're struggling to see a profit - but I'm told that in some cases that's how it starts out, but ultimately you can make organic non-ad sales once your book has had enough sales & reviews for Amazon to start showing it a bit more liberally in the natural search results. Of course it matters a ton what your title, subtitle, and keywords are for your product - and ad, if you choose to run one.
Pinterest and social media are other reportedly good advertising venues. It's quite amazing to me what ChatGPT can do with a set of rules surrounding puzzles - but honestly, I have put TONS of work into it. The work, re-work, re-do's, spot checking, and just re-doing everything until it comes out right can be mind numbing, but once you start to learn what chatty can and can't do and know just when to give it which reminders and parameters, you can create a 100 puzzle book in a few minutes - complete with matching solutions in the back - IF you've suffered through the prerequisite learning phases first.
So far I haven't quite broke even yet, but I'm investing in some ad money to get some reviews and visibility. Once that happens I'll see if I can make more organic sales and/or focus my ad money on 1-3 books (out of the 70+ I have out there, most of which were experimental in hindsight).
Blood sweat 'n tears but it's a fun hobby so far.
