guyeshet

function register_book_post_type() {

	$args = array (
		'label' => esc_html__( 'Books', 'text-domain' ),
		'labels' => array(
			'menu_name' => esc_html__( 'Books', 'text-domain' ),
			'name_admin_bar' => esc_html__( 'Book', 'text-domain' ),
			'add_new' => esc_html__( 'Add new', 'text-domain' ),
			'add_new_item' => esc_html__( 'Add new Book', 'text-domain' ),
			'new_item' => esc_html__( 'New Book', 'text-domain' ),

guyeshet

<?php

add_action( 'mb_relationships_init', function() {

    MB_Relationships_API::register( array(
        'id'   => 'books_to_authors',
        'from' => array(
            'object_type'  => 'post',
            'post_type'    => 'book',
            'admin_column' => 'before title',  // THIS!