add_filter( 'rest_gallery_query', function( $args ) {
  $args['meta_query'] = array(
    array(
      'key'   => 'book_id',
      'value' => esc_sql( $_GET['book_id'] ),
    )
  );

  return $args;
} );

add_filter( 'rest_gallery_query', function( $args ) {
    if ( trim($_GET['book_id']) ) {  // check if book_id is passed and is not empty, but you can modify this condition
        $args['meta_query'] = array(
            array(
                'key'   => 'book_id',
                'value' => trim( $_GET['book_id'] ),  // <- you don't need to esc_sql this
            )
        );
    }

    return $args;
} );