So this category is on one page, but other pages didn't get this even though they have ISBN numbers. Is there a way to get rid of this category but allow the ISBN links or at least add ISBN links to all book pages?
It's a tracking category that was activated when ISBN extension was fixed last week-ish. There's really no way to get rid of it except creating it as a hidden category.
The other pages will show up in there too, once they've had an edit or null edit.