Another interesting shop I've discovered for me within the last year is itch.io. It's mostly for games, but I found out that this includes more and more tabletop role-playing games as e-books. The site is responsive, the individual "store fronts" often have unique customizations, and payment/downloads works reliably. And the RPG PDF market is similarly dominated by a single entity (drivethrurpg), so getting away from that feels good, too.
They also feature an interesting revenue sharing option, where you're deducated the payment processor fees (stripe,paypal) directly, but after that the amount you pay to itch.io is totally customizable (and even their default is just 10%).
There's also a culture of "pay what you want" publications and people paying more than what's asked for. This is partly due to publications often being smaller and more frequent, instead of big honking monographs. As opposed to the programming/entrepreneur market, where e-books often seems ridiculously overpriced for what you get ("here's slightly value-added rephrasing of the react tutorial, that's $49, please").