The Eyes of Darkness by Dean R Koontz

Tina Evans is an ex Vegas showgirl opening her first major show on the Strip and slowly coming to terms with the death a year earlier of her 12 year old son in a terrible crash with his scout troop in the high Sierra mountains, a crash that left his body so mangled she and her husband were advised to have a closed coffin ceremony.  Tina is now living alone, Danny's room remains unchanged and one night she hears a heavy thump from his room.  She finds Danny's blackboard overturned, on it are written two words: Not Dead.  Initially Tina believes it's a sick joke perpetrated by her ex-husband, but as the incidents increase in intensity and complexity, always saying that Danny is not dead, is in pain and needs her, she finally becomes convinced Danny is indeed not dead.  With attorney she heads for the mountains and, as often with Koontz, the answer lies underground in a secret lab where decisions are made by evil men for the 'good of the nation'.   Although this is an early Koontz novel familiar themes of a hostile government, the propensity of evil to thrive in secrecy and the benevolence of mysterious forces are already here.