[Fixed]-Django: How to modify the same field in other entries sharing the same Foreign Key?

Quite simply, you don’t store the latest book, you can just have it as a method or property of the author that can do a query to retrieve it

def latest_book(self):
    return self.book_set.order_by('published_date').first()

Storing logic into your database is a bad idea, it should only store data.