from django.contrib.contenttypes.models import ContentType
from django.db.models import Prefetch
book_ct = ContentType.objects.get_for_model(Book)
TaggedItem.objects.filter(content_type=book_ct).prefetch_related(Prefetch(
    'content_object', queryset=Book.objects.all().select_related('author')))
Is django prefetch_related supposed to work with GenericRelation