This might be a heavy/impossible lift given the current data model, but doesn't hurt to ask... There are cases of "box sets" or other collected works that are multiple books collected under a single ISBN. Unfortunately, each individual book has the same ISBN and there is no way to distinguish the individual books within the set, even if they're clearly distinct.
E.g.
9780063392939 - lotr box set
9781632364616 - akira box set
9780740748479 - calvin & hobbes

Ideally there'd be some way to have separate editions for the individual books that are still tied to the same isbn, or having some "version" of the single isbn edition that can be linked and tracked with the individual book entries.

Also, having the box sets themselves represented as a single "book" isn't really accurate and might make sense to have these represented as a separate category in itself. There are examples of box sets that have unique ISBN's for the box set itself, but the individual books still have their own unique ISBN - less of an issue, but treating the two as separate but linked would still be nice. E.g. 9780321751041 - art of computer programming box set, but the individual books have different isbn's.