I'm not much of a writer, so this will be short.
Recently, I decided to change the operating systems on my devices, including the smartphone through which I used to read ebooks bought from Amazon and Kobo. There's no need to mention that both Android apps are proprietary.
I flashed LineageOS to my phone and F-Droid helped me find many open source replacements for the apps I used to have. I'll name a few: andOTP (a reliable 2FA authenticator), OpenVPN for Android (manually configured for PIA, successfully skipping the proprietary app) and Ring (for online messaging). LineageOS already provides some basic apps out-of-the-box, and I kept them.
Kobo and Kindle virtual books are tightly protected and cannot be read by any other application besides the official ones. You could imagine they would provide you with a copy in PDF format if you already bought it, but that's not the case. You cannot read your books any longer, because you are not using the operating system THEY WANT or expect you to use.
If you're on GNU/Linux or a custom ROM without Google's touch, you're completely ignored.
That is the problem with non-existent/virtual products. You can never really own them. This trend advances to take power from users, and I strongly advise to not feed it.
I guess I'll go back to physical books while I don't find a better alternative.
RELATED THINKING: What if our government-issued IDs and services become digital as well? Will we need to own a "certified device" that cannot be tampered or have its ROM flashed in order for the ID to be considered valid? Think about it.
I just made an account at archive.org and here's my first contribution: Sleep State Interrupt, T.C. Weber