add_rewrite_rules for type of post

by

the URL index.php?pagename=to-kill-a-mockingbird&post_type=book&book_type=fiction works only for simple post or page.
when you have a custom post type, the URL is index.php?book=to-kill-a-mockingbird&book_type=fiction.

then try this code : 

add_rewrite_rule(
      '^books/fiction/([^/]*)/?$'
    , 'index.php?book=$matches[1]&book_type=fiction'
    , 'top'
);