使用ModelForm和ModelMultipleChoiceField保存到许多tomanyfield

I've created a basic Django app that contains books/authors/publishers as per the Django Book - trying to use a ModelForm to create a means to modify existing books - the problem is that the 'authors' field is a ManyToManyField and when I choose a choice on the ModelForm it simply wipes the existing selection and doesn't save the new one?I've created a basic Django app that contains b