Nurse Sarah Keate, the amateur snoop who made readers of "The Patient in Room 18" and "While the Patient Slept" shudder, is back in stand-up-to-the-devil form. Now she supervises the third floor of a metropolitan hospital. From the landings of a walnut stairway, unlighted for the sake of economy, she can see everything going on. She generally uses the stairs instead of the lurching cagelike elevator, but one sweltering summer night-- ! Let her tell...