get chapters for relevant book

functions.php

function sdesigns_ajax_bookblock_function(){
    $download_id = $_POST['id'];
    $output = '';

    if ( $_POST['sameIds'] ) {
    	$args = array(
			'post_type' => 'book',
			'posts_per_page' => -1,
			'order' => 'ASC'
		);
    } else {
    	$args = array(
			'post_type' => 'book',
			'posts_per_page' => -1,
			'order' => 'ASC',
			'meta_key' => 'chapter_belongs_to',
			'meta_value' => $download_id
		);
    }