alt Hacker NewsHardcover and ISBNDB have a good amount of data, with Hardcover being excellent for getting good covers and genres.
I'm hoping Goodreads and Anna's Archive will help fill in the gaps, especially since Anna's Archive have gigantic database dumps available[1].
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