How to add custom table data to Relevanssi's search index

acfcdt-relevanssi-support.php

<?php

add_filter( 'relevanssi_content_to_index', 'acfcdt_relevanssi_support', 10, 2 );
add_filter( 'relevanssi_excerpt_content', 'acfcdt_relevanssi_support', 10, 2 );
function acfcdt_relevanssi_support( $content, $post ) {

	/**
	 * Approach A (recommended): Using SQL to minimise database queries during Relevanssi's indexing process.
	 */
	global $wpdb;
	$data = $wpdb->get_results( "SELECT `book_name`,`book_year` FROM `wp_bookmeta` WHERE post_id = $post->ID", ARRAY_A );
	if ( ! empty( $data[0] ) ) {
		$values  = array_values( $data[0] );
		$content .= join( ' ', $values );
	}

	/**
	 * Approach B: Using ACF's get_field() function on individual fields
	 */
	//$content .= get_field( 'book_name', $post->ID );
	//$content .= get_field( 'book_year', $post->ID );

	return $content;
}