The Dark Door — Kate Wilhelm

Charlie Meiklejohn and Constance Leidl were an unlikely pair of detectives. He was a former arson investigator, she was a psychologist, and the two were looking forward to the quiet relaxation of country life. But Charlie and Constance had a knack for unraveling mystery--and mystery always seemed to find its way to their door.

Someone was setting fire to abandoned hotels, restaurants and schools around the country, at erratic intervals, and in apparently arbitrary locations. Worse, each time a building was torched, the arson was accompanied by madness, murder and mutilation. Charlie and Constance would have to call upon all their skills to discover what lay behind this random insanity. What they would discover was far more terrifying than they could imagine....