I've made 2 custom birdwatching checklist books (NY and UK)
This probably doesnt have a big market, especially because it's niche inside a niche, but it's my hobby and I enjoyed making these and hope to make more.
They're intended to be kind of like Pokedexes for birdwatchers. Each takes about 2 months to finish.
NY Version:
https://www.etsy.com/listing/4483233692/new-york-bird-checklist-bird-watching
UK Version
https://www.etsy.com/listing/4505978074/uk-bird-watching-checklist-life-list-log
EDIT: I see that people feel very strongly about the use of AI in this book so I want to say a few thing about that.
First: This is book is not art and is not advertised as such. This book has didactic and practical purposes. The images are there to show what the birds look like, not to flex my art skills.
Second: I didn't just prompt the AI to make me a book. I spent over 200 hours on each of these books, a big part was with the images sure but there was also lot's of research involved to make sure each classification is accurate and true and to make sure each bird belongs in the list and there's no birds missing. And I didn't just ask the AI to do this for me, I actually manually researched each bird individually on public databases to check how often they are sighted, and official channels to know what the correct status and classification is. I'm a birder, not an artist.
It would've been a lot less effort to just use images from a public database, but the focus of the book is not the images, it's the practical purpose it serves for birders. I made this for me at first and I thought other people might like it as well.