Annoying to set up but functional
I’ve been a kindle reader for years. Use ipad kindle app and cloud reader all the time.
I had some trouble with set up; it seems like it silently fails? If you can’t see your books this may help.
(Change your password on the amazon site, Deregister from the preferences menu option, reregister with your new password).
Not sure which part of that resovled the issue but my books are available.
znodd about Kindle Classic