Does openlibrary.org support Z39.50 access?
The way the openlibrary.org site is constructed seems better than the others that came before, e.g., WorldCat. Every page is not just HTML but JSON, too. Omit the slug and add .json after the ID.
https://news.ycombinator.com/item?id=25498127
This is without even considering the availability of bulk data dumps. A+