function save_book_meta( $post_id, $post, $update ) {

    $slug = 'book'; //Slug of CPT

    // If this isn't a 'book' post, don't update it.
    if ( $slug != $post->post_type ) {
        return;
    }

    wp_set_object_terms( get_the_ID(), $term_id, $taxonomy );
}

add_action( 'save_post', 'save_book_meta', 10, 3 );