I see you have your catalog data in WorldCat: https://www.worldcat.org/libraries/58122

But WorldCat says there are only 10,136 items.

So, what data do you have that isn't already in there, and can you get it in there? Most developers I know that care about libraries are querying WorldCat.

Work with the Internet Archive to get your data included into https://openlibrary.org/

Once that work has been done, you can put your own API on the same data to address use-cases WorldCat and OpenLibrary don't (or don't easily).