Booksearch

Note: Open Library + Google Books run directly. ISBNdb/VLB need a key/token (and possibly a backend because of CORS).

Options: Google Books

Without a key you may hit limits sooner; with a key it is more stable.

Options: ISBNdb (commercial)

May fail in the browser because of CORS → use a proxy/backend then.

Options: VLB (auth required)

VLB is great for German-language publisher data, but always requires authentication.