Parent & Child Taxonomy slugs in custom post permalink

Question

To handle the extra URL segment you need to add another rewrite rule similar to the one generated by add_permastruct, but with an extra capture group and slash for the additional level of terms:

add_rewrite_rule(
    'book/([^/]+)/([^/]+)/([^/]+)-blah-blah-blah?(:/([0-9]+))?/?$',
    'index.php?post_type=book&name=$matches[3]',
    'top'
);

Also, as I mentioned in my comment, the query var and slug author are already in use by core, you should probably change those.

Also in your post_type_link function, you should check if terms/parent terms exist before trying to access their properties, which will produce warnings with debugging enabled.