[Quoth]

D&T, re: Amazon
Go to Amazon Website and login.
Go to Content and Devices (name varies with MarketPlace)
Go Books
select more under the button on row with a title
1st Option is Delete
2nd Option is Download and Transfer via USB (or similar)
You have to then select a registered Kindle (or maybe some Fire models but not an App), even if there is only one.

D&T is downloading using a browser. Works on ANY OS that supports javascript browser, Saving to local storage and later ability to browse that and copy to the Kindle selected via USB. Trivial on Windows, mac OS and Linux and works on Android with an adaptor on USB port. Maybe on iOS too.
The file(s) mostly only work on the Kindle you selected unless you use the NoDRM/DeDRM plug-in on Calibre.

I've had 5 different models of Kindle and also tried a PW4. Still have 3 different Kindles, used for test, or purchasing an ebook from Amazon, but maybe 90% of my ebooks are not from Amazon.
I've also used / had a Nook, iRiver Cover Story, 3 models of Sony, a reMarkable (which isn't), an android eink, a Y-Ben P47L and even 3 models of LCD based dedicated Binatone.
I've also used eBooks on Palm OS, Symbian and Android apps.

Kobo is the best, and I find of the 6 models of Kobo I have or had that the best all round one is the Sage 8″. I never use WiFi on any of my ereaders.