Fair enough, but my issue with your original comment is that it implied that _by design_, ISBN does not uniquely identify books, which is wrong, as far as I can

Fair enough, but my issue with your original comment is that it implied that _by design_, ISBN does not uniquely identify books, which is wrong, as far as I can tell. In any case, the issue of reuse of ISBN has practical consequences, so that the original intent might not be so important in the end. Do you have any idea how many cases there are of reuse? Is it actually common?