[ownedbycats]

Personally, I use the kobo: identifiers (https://www.kobo.com/ca/en/ebook/{id}) used by the older Kobo Books plugin. of course it does have its own problems, e.g. the number at the end changing when the book's edition gets updated.

Unfortunately, since the passing of davidfor I've found that plugin failing more and more often, presumably due to site changes, which is why I was looking at this as a replacement (or supplement).