Show Custom Taxonomy Panel in the Gutenberg Editor

books.php

<?php
/**
 * Adds Books to your website.
 *
 * @package      KnowTheCode\Books
 * @author       hellofromTonya
 * @license      GPL-2.0+
 *
 * @wordpress-plugin
 * Plugin Name:       Books
 * Plugin URI:        https://KnowTheCode.io
 * Description:       Adds books to your website.
 * Version:           1.0.0
 * Author:            hellofromTonya
 * Author URI:        https://KnowTheCode.io
 * Text Domain:       books
 * License:           GPL-2.0+
 * License URI:       http://www.gnu.org/licenses/gpl-2.0.txt
 * GitHub Plugin URI: https://KnowTheCode.io
 * Requires PHP:      5.6
 * Requires WP:       4.9
 */

namespace KnowTheCode\Books;

add_action( 'init', __NAMESPACE__ . '\register_book_post_type' );
/**
 * Register a Book custom post type.
 *
 * @since 1.0.0
 */
function register_book_post_type() {
	$labels = [
		'name'               => _x( 'Books', 'post type general name', 'books' ),
		'singular_name'      => _x( 'Book', 'post type singular name', 'books' ),
		'menu_name'          => _x( 'Books', 'admin menu', 'books' ),
		'name_admin_bar'     => _x( 'Book', 'add new on admin bar', 'books' ),
		'add_new'            => _x( 'Add New', 'book', 'books' ),
		'add_new_item'       => __( 'Add New Book', 'books' ),
		'new_item'           => __( 'New Book', 'books' ),
		'edit_item'          => __( 'Edit Book', 'books' ),
		'view_item'          => __( 'View Book', 'books' ),
		'all_items'          => __( 'All Books', 'books' ),
		'search_items'       => __( 'Search Books', 'books' ),
		'parent_item_colon'  => __( 'Parent Books:', 'books' ),
		'not_found'          => __( 'No books found.', 'books' ),
		'not_found_in_trash' => __( 'No books found in Trash.', 'books' ),
	];

	$args = [
		'labels'             => $labels,
		'description'        => __( 'Cool books.', 'books' ),
		'public'             => true,
		'publicly_queryable' => true,
		'show_ui'            => true,
		'show_in_menu'       => true,
		'show_in_rest'       => true, // To use Gutenberg editor.
		'query_var'          => true,
		'rewrite'            => [ 'slug' => 'book' ],
		'capability_type'    => 'post',
		'has_archive'        => true,
		'hierarchical'       => false,
		'menu_position'      => null,
		'supports'           => [ 'title', 'editor', 'author', 'thumbnail', 'excerpt' ],
	];

	register_post_type( 'book', $args );
}

add_action( 'init', __NAMESPACE__ . '\register_genre_taxonomy', 0 );
/**
 * Register the Genre custom taxonomy.
 *
 * @since 1.0.0
 */
function register_genre_taxonomy() {
	$labels = [
		'name'              => _x( 'Genres', 'taxonomy general name', 'books' ),
		'singular_name'     => _x( 'Genre', 'taxonomy singular name', 'books' ),
		'search_items'      => __( 'Search Genres', 'books' ),
		'all_items'         => __( 'All Genres', 'books' ),
		'parent_item'       => __( 'Parent Genre', 'books' ),
		'parent_item_colon' => __( 'Parent Genre:', 'books' ),
		'edit_item'         => __( 'Edit Genre', 'books' ),
		'update_item'       => __( 'Update Genre', 'books' ),
		'add_new_item'      => __( 'Add New Genre', 'books' ),
		'new_item_name'     => __( 'New Genre Name', 'books' ),
		'menu_name'         => __( 'Genre', 'books' ),
	];

	$args = [
		'hierarchical'      => true,
		'labels'            => $labels,
		'show_ui'           => true,
		'show_admin_column' => true,
		'show_in_rest'      => true, // Needed for tax to appear in Gutenberg editor.
		'query_var'         => true,
		'rewrite'           => [ 'slug' => 'genre' ],
	];

	register_taxonomy( 'genre', [ 'book' ], $args );
}