HAVE YOU EVER purchased a self-help book? If so, you are like most Americans. In 2003 alone, publishers put out more than 3,500 new self-help titles, ringing up more than $650 million in sales. Many of the buyers cannot or will not seek psychotherapy, but surveys by John C. Norcross of the University of Scranton and others indicate that 80 percent or more of psychotherapists recommend such books to their patients, too. How well are self-help books fulfilling their purpose?