Ask HN: Where to buy (DRM-free) ePub books?

So I just purchased my non-kindle e-reader. I would like to buy books for it. Like the shelf books, but to my e-reader.

Do such websites to legally acquire e-pub books even exist? I wasn't able to find one. Happy to pay for those as much as for physical copies.

I am interested in history and politics; my first target is 0199797455, assuming I can find where to buy it.