You just need to figure out the file to which the ebook was stored. As an example, in my case that was `~/.config/chromium/Default/databases/https_lesen.amazon.de_0/1`
If you have installed the browser as snap package, you need to first check where that snap is mounted (e.g. with `mount | grep snap`).
Go to the `library` view in the cloud reader App, right click the cover, click `Download` (if you don't have that option, you may not have the cloud reader extension installed).
If an ebook was opened in the browser, but not downloaded, the sqlite file will exist, but not contain the content necessary for export (and this script will thus fail).