Custom taxonomy registration code for the “Books” Custom Post Type.

taxonomy-registration.php

<?php // Ignore this line if your functions.php already include it.
function register_book_author_taxonomy() {
    $labels = array(
        'name'                       => 'Book Authors',
        'singular_name'              => 'Book Author',
        'menu_name'                  => 'Book Authors',
        'all_items'                  => 'All Book Authors',
        'edit_item'                  => 'Edit Book Author',
        'view_item'                  => 'View Book Author',
        'update_item'                => 'Update Book Author',
        'add_new_item'               => 'Add New Book Author',
        'new_item_name'              => 'New Book Author Name',
        'parent_item'                => 'Parent Book Author',
        'parent_item_colon'          => 'Parent Book Author:',
        'search_items'               => 'Search Book Authors',
        'popular_items'              => 'Popular Book Authors',
        'separate_items_with_commas' => 'Separate book authors with commas',
        'add_or_remove_items'        => 'Add or remove book authors',
        'choose_from_most_used'      => 'Choose from the most used book authors',
        'not_found'                  => 'No book authors found',
    );

    register_taxonomy( 'book_author', 'books', array(
        'labels'            => $labels,
        'hierarchical'      => true,
        'public'            => true,
        'show_ui'           => true,
        'show_admin_column' => true,
        'show_in_nav_menus' => true,
        'show_in_rest'      => true,
        'query_var'         => true,
        'rewrite'           => array( 'slug' => 'book-author' ),
    ));
}
add_action( 'init', 'register_book_author_taxonomy' );