Who can help me remember a book by the plot only?

I read a book many years ago and cannot remember the title or author. The plot involves a psychiatrist/psychologist who is hypnotizing his patients in the office unknowingly and then he manipulates them by calling them on the phone and using a word that is the trigger to send them into a hypnotized state, one girl (patient) dies and her friend starts unraveling the clues to figure out the doctor is doing this and then he is after the friend trying to kill her. Any avid book readers who remember this book pleaseeeee let me know !!! I have tried all the websites dedicated to finding a book by plot and have been unable to find it, hope someone out there has read this book and remembers it ! THANKS !!