Custom post type as child of page

If you want your custom post type listed under a page you can add the permalink of the page to the rewrite slug like so:

register_post_type('my_books', array(
'has_archive' => false,                 // Stops domain.com/book becoming the cpt's archive
'rewrite'     => array('slug' => 'book')
));

You need to visit Settings->Permalinks after making any changes to permalinks to re-generate them. You should now be able to visit books under mysite.com/book/my-book-slug and visit your manually created Page under mysite.com/book.