

According to the terms, when you purchase a Kindle e-book, you are buying a license to access the content rather than owning the book outright. And the only reason they made it explicit is CA law AB 2426. So you can “access” it on any device that can display their content, be it an app or hardware device, but you can’t possess it via a download for example. (I find this all to be bullshit, I’m just stating Amazon’s position on the topic)
This is a big part of why I have a kobo, the files are easy to scrub of the DRM but I’m still getting an easy way to throw money at creators I value.
I don’t have an easy answer. Amazon was the 1 ton gorilla in the room so that was the one I was familiar with, but I will try to update this comment as I find information out.
Kobo: Seems like it’s yours to keep without resale or transfer rights. (https://download.kobobooks.com/learnmore/kobo1_pdf/Kobo_eReader_Terms_of_Use.pdf)
Google: Yours to keep provided the authorized agent and Google themselves maintain rights to provide it. (https://books.google.com/intl/en/googlebooks/tos.html)
B&N Nook: seems to be more in line with Amazon (https://www.nook.com/services/cms/doc/us/en_us/legal/nook-store-terms.html#ItemsPurchases)
Smashwords: seems more like ownership without transferability. (https://www.smashwords.com/about/tos)