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 prepares the text to be displayed in the browser's title bar:

add_filter( 'document_title_parts', 
            'ch4_br_format_book_review_title' ); 

4. Add the following code section to provide an implementation for the ch4_br_format_book_review_title function:

function ch4_br_format_book_review_title( $the_title ) { 
    if ( 'book_reviews' == get_post_type() && is_single() ) { 
        $book_author = esc_html( get_post_meta( get_the_ID(),  
                                   'book_author', true ) );
        if ( !empty( $book_author ) ) {
            $the_title['title'] .= ' by ' . $book_author;
        }
    } 
 
    return $the_title; 
} 

5. Save and close the plugin file.

6. Visit a Book Review page. You will see that the book's author is now displayed after the name in the title: