

Thanks for asking this, that question made me go look it up.
I found https://bookshop.org/info/ebooks
I thought you could load bookshop.org eBooks onto a Kindle, but it seems they have their own DRM and you need to use their own app…
Some of their books are DRM free, but not all. I thought they all were, but it turns out I was wrong.
So… maybe even bookshop.org isn’t the best option for Kindles.
I guess there really is only one option left…
Note-taking app. Each note is a markdown file, so you can add formatting.