<?php
add_action( 'init', 'create_book_tax' );

function create_book_tax() {
    register_taxonomy(
        'genre',
        'page',
        array(
            'label' => __( 'Genre' ),
            'rewrite' => array( 'slug' => 'genre' ),
            'hierarchical' => true,
        )
    );
}
?>