Pretty sure it was by Dean Koontz but not 100% sure. A guy is a villain/therapist who preys on people and makes them think they have mental illnesses. He convinced a lady she has agoraphobia. The main character and her husband (Dusty Rhoades i think was his name?) Investigate the guy but he tries getting into their heads too.
Lost Classics of Teen Lit: Name That Book! 
It is Dean Koontz, and the title is “False Memory.”
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