ISBN is not unique; sometimes ISBNs are reused if books are uncatalogued (or for some other reason). If you sell a book in Amazon (or other self-publishing platforms), you might request an ISBN or not. They have an unique ID, however, which they call ASIN. Read all about ISBNs here
So compulsorily using a ISBN in all books might need some consideration.
https://github.com/Anglepi/My-Many-Reads/blob/5cbbdb465e96b3b8084626d80d1976b77c2d8593/mmr-backend/book.py#L9
ISBN is not unique; sometimes ISBNs are reused if books are uncatalogued (or for some other reason). If you sell a book in Amazon (or other self-publishing platforms), you might request an ISBN or not. They have an unique ID, however, which they call ASIN. Read all about ISBNs here
At any rate, identifying a book with an (external) ID is not trivial, there are several ways to represent it.