MyBookTable: use book image as post thumbnail

functions.php

/**
* 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;
	}

	if ( is_admin() ) {
		return $null;
	}
	
	if ( get_post_type( $object_id ) == 'mbt_book' ) {

	    remove_filter( 'get_post_metadata', __FUNCTION__, 10, 4 );

	    $featured_image_id = get_post_meta( $object_id, 'mbt_book_image_id', true );

	    add_filter( 'get_post_metadata', __FUNCTION__, 10, 4 );

	    // The post has a featured image.
	    if ( $featured_image_id ) {
	    	return $featured_image_id;
    	}

	}

	return $null;
}
// Set MBT Thumbnail size to Large
add_filter( 'post_thumbnail_size', 'mbt_post_thumbnail_size', 10, 2);
function mbt_post_thumbnail_size( $size, $post_id ) {
    
    if ( get_post_type( $post_id ) == 'mbt_book' ) {
            $size = 'large'; // Change to your desired image size.
    }

    return $size;
}

This code returns the MyBookTable Book Image ID when theme & plugins are calling the has_post_thumbnail or get_the_post_thumbnail functions for the mbt_book custom post type.

This is brilliant :) just used it and it worked like a charm