Ok, thanks - now everything launches fine.
But still gui doesn't change.
Code:
mi = self.gui.library_view.model().db.get_metadata(rows[0].row())
print(self.db.new_api.field_for('title', book_id))
self.db.new_api.set_field('title',{book_id:'STH'})
self.gui.iactions['Edit Metadata'].refresh_books_after_metadata_edit({book_id})