contemplate

/**
* MyBookTable: use book image as post thumbnail
*/
//Find Book Image ID
add_filter( 'get_post_metadata', 'mbt_use_book_image_as_thumbnail', 10, 4 );
function mbt_use_book_image_as_thumbnail( $null, $object_id, $meta_key, $single ) {
	if ( '_thumbnail_id' !== $meta_key ) {
		return $null;
	}