Ive found some older databases may have orphaned address_book entries due to not properly removing customers or whatever reasons - here are some snippets to clean up the address_book table.
First, run this query and if you have any records returned, create a view
SELECT * FROM address_book a WHERE NOT EXISTS (SELECT 1 FROM customers c WHERE c.customers_id = a.customers_id)
ORDER BY a.customers_id;
~Do a google for creating a view in phpMyAdmin ~ lots of info exist, but you'll find the create view button at bottom of the page; name it something like orphaned_address_books.
It is now up to you to confirm the integrity of the data returned, look at some of the customers_id's in the view, check to see if the same customers_id exist in the customers table - if they DO NOT exist in the customers table then you may proceed. You don't really want to waste time checking each of them if you have a high return, just randomly select a handful to ensure the query returned as expected.
Now, using the view you should be able to delete the orphaned rows.
By the way ~ YOU did backup the address_book table, right?