This add-on does not work.

First of all, it recognizes only the ISBN-13 format, not ISBN-10 (whereas we often still might encounter ISBN-10 format).

Secondly, and more importantly, the whole search functionnality of this add-on relies on sending the search request to a Nginx server on the domain name "isbnsearcher.com" which seems to be their own. (same domain name in the add-on homepage)

However, whatever treatment of the query was meant to be on this server, or redirection towards other third-part ISBN search engines, this domain/server is down or not functional for the treatment of the requests. All queries tried replied with "404 error - not found "

The homepage isbnserver.com only displays "webserver", so it seems to be up, but not taken care of.

Bad idea to throw queries at one's own server rather than to some more stable ISBN search engines out there.