`fetch-ebook-metadata` is not a CLI for the library, it is a CLI for individual books.
- You can use ebook-meta or `calibredb show_metadata $id` or read the OPF file, in order to get the current title/author/ISBN for a book.
- Then `fetch-ebook-metadata` can retrieve new metadata from online, and print it to OPF format.
- `ebook-meta` or `calibredb set_metadata` can then save the retrieved metadata using the OPF from step two.