You should also consider OpenLibrary and LibraryThing. Both of which have good coverage on WikiData which also aggregates identifiers.

In fact, now that I think about it, you could also contribute your work to WikiData. I don't see ISBNdb ids on WikiData so you could write a script to make those contributions. Then anyone else using WikiData for this sort of thing can benefit from your work

I haven’t created a ticket for OpenLibrary yet, but it’s on my mental todo list. I’ll create the ticket for multiple new extractors today.

I’d love to help improve other services. I plan on charging for Librario at some point, but I’ll offer a free version and offer free API keys for projects like Calibre and others.

At least that’s the plan.

ISBNid seems to use ISBN-13 for its unique identifier, which is just Property:P212 on Wikidata.