Adding on to this that Barnes and Noble sells DRMed ePUB files that are relatively easy to strip DRM from using Calibre.

So if you can’t find a book anywhere else, at least they don’t use a proprietary format and still allow you to download your books using their PC software.

I was a semi-early adopter in the ebook space and I have refused to get onboard with the kindle ecosystem from the start. There’s no reason for their proprietary format other than complete control over things they pretend to sell you. Amazon is also the Walmart of books and uses their position to browbeat publishers and authors into taking smaller cuts of sales.