1. Navigate to the ch4-book-reviews folder of the WordPress plugin directory of your development installation.
2. Open the ch4-book-reviews.php file in a code editor.
3. Add the following line of code after the existing functions to register a function to be called when WordPress is preparing the filter drop-down boxes for the post listings:

add_action( 'restrict_manage_posts',
            'ch4_br_book_type_filter_list' ); 

4. Add the following code section to provide an implementation for the ch4_br_book_type_filter_list function:

function ch4_br_book_type_filter_list() { 
    $screen = get_current_screen(); 
    global $wp_query; 
    if ( 'book_reviews' == $screen->post_type ) { 
        wp_dropdown_categories( array( 
            'show_option_all' =>  'Show All Book Types', 
            'taxonomy'        =>  'book_reviews_book_type', 
            'name'            =>  'book_reviews_book_type', 
            'orderby'         =>  'name', 
            'selected'        =>  
            ( isset( $wp_query->query['book_reviews_book_type'] )         
              ? $wp_query->query['book_reviews_book_type'] : '' ), 
            'hierarchical'    =>  false, 
            'depth'           =>  3, 
            'show_count'      =>  false, 
            'hide_empty'      =>  true, 
            ) ); 
    } 
} 

5. Insert the following line of code to register a function that will be called when the post display query is being prepared:

add_filter( 'parse_query', 'ch4_br_perform_book_type_filtering' );

6. Implement the ch4_br_perform_book_type_filtering function with the following code segment:

function ch4_br_perform_book_type_filtering( $query ) {
    $qv = &$query->query_vars; 
 
    if ( isset( $qv['book_reviews_book_type'] ) &&
         !empty( $qv['book_reviews_book_type'] ) && 
         is_numeric( $qv['book_reviews_book_type'] ) ) { 
        $term = get_term_by( 'id', 
                             $qv['book_reviews_book_type'], 
                             'book_reviews_book_type' );     
        $qv['book_reviews_book_type'] = $term->slug; 
    } 
} 

7. Save and close the plugin file.
8. Visit the Book Reviews listings to see the new dropdown to restrict what book types are displayed: