@dangillmor these downloads are tied to a single device so backing them up is of questionable utility. the only reason anyone cares about this is because the drm on them has been broken for a very long time so most people breaking drm were still using it
but as of a few years ago you can also just download books on-device and then import them into calibre over usb, or with a bit more setup download books using the kindle desktop app and import into calibre from there (which also gives you full-resolution images, unlike downloads for e-readers)