@helmut_meukel
Assume you want to replace your computer. Can you simple copy or move the folder containing your Kindle books to the new computer? Can you install the reader program on the new computer without a connection to Amazon?
For Kindle Reader for PC, yes (I've done it).
For a Kindle paper white, yes, but it's a multi step pain in the ass process.
For Kindle for Android, maybe, if you are configured to store your Kindle books on an SD card.
But how when the computer is broken down? Retrieve them from a backup?
Depends on the nature of the break down.
When I replaced my last computer, to save the time and cost of having the data on my old computer's 2TB drive copied to the new computer, I had the shop install the drive from my old computer on the new one as a secondary drive.
IIRC, you can then import the old library files into the new install.
As long as the drive on the old computer is good, you could do something similar.
How to replace a Kindle device? (without Amazon, you may get a used Kindle, but how to transfer the books?).
Dedicated Kindle readers (the paper white devices) don't support removable media. However, it is possible to access the storage on a Kindle by plugging it into a PC. It will see the Kindle as a USB drive. It is possible to manually load e-books on a Kindle reader this way.