Add custom column to post list table

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.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)