Hi,
Would it be to difficult to had an other source of metadata ?
i'm thinking about the BNF (Bibliothèque nationale de France) for all the books in French. The French National Library have a pretty fast and usefull SRU :
https://api.bnf.fr/fr/api-sru-catalogue-general
Could give a hand to help add the BNF as metadata sources even if i'm not a professional in coding
