Maybe you just need to change it to:
case 'book_author' :
$value = get_field( "contact_person", $post_id );
The second parameter is probably needed as it is in a loop, but not The Loop
.
Maybe you just need to change it to:
case 'book_author' :
$value = get_field( "contact_person", $post_id );
The second parameter is probably needed as it is in a loop, but not The Loop
.