The Eyes of Darkness by Dean Koontz

Tina Evans has spent a year suffering from incredible heartache since her son Danny's tragic death. But now, with her Vegas show about to premiere, Tina can think of no better time for a fresh start. Maybe she can finally move on and put her grief behind her. Only there is a message for Tina, scrawled on the chalkboard in Danny's room: NOT DEAD. Two words that send her on a terrifying journey from the bright lights of Las Vegas to the cold shadows of the High Sierras, where she uncovers a terrible secret..