I found an adequate solution.
Generally, I used the save_post_{$post->post_type}
hook which gave me a $post_ID
and a $post
object.
I was just checking to see if there was a thumbnail and if is not there to add it. That is all.
add_action( 'save_post_book', function ($post_ID, $post){
if ( get_post_meta($post_ID, '_thumbnail_id', true) ) :
return;
endif;
if($thumbnail_id = get_option('my_default_thumbnail_id', false)) {
add_post_meta($post_ID, '_thumbnail_id', $thumbnail_id);
}
}, 10, 2);