I'm a huge fan of Pia. I think Facing Codependence is the best book to start with. They definitely go in order. There was a time when I was listening to a set of her tapes over and over while I was chopping dried fruits for hours (I think it was about co-addicted relationships). Always a bracing and inspirational kick in the arse. Her voice would come to me with key instructions at key moments: "Who she is is none of your business!"
Actually, a friend just loaned me an old Walkman so I could listen to those tapes again. It's time for a refresher.