INNOCENT BLOOD
by P.D. James
Scribners; 311 pages; $10.95
Over the past decade or so, P.D. James has become perhaps the best living writer of traditional mysteries. She has, of course, the necessary virtue of strong and resourceful plotting, but two far more remarkable qualities mark her fiction. One is a skill in writing about white-collar work. James is almost 60 now, and has spent her life in the British civil service. She understands the lower and middle levels of bureaucracy, and she takes the nurses, supervisors, administrators and the people who fetch their...