A new mystery from Baroness P. D. James is always a cool, elegant, meticulously plotted treat, though her latest moves a bit more slowly than her sparkling early work. A famous novelist is found hanging outside a Lighthouse on the tiny Cornish island of Combe, a private retreat for distinguished public servants. Commander Adam Dalgliesh helicopters in to investigate and discovers no shortage of islanders with motivation to off the irascible writer. James' decision to stage the murder in a closed community a classic Agatha Christie technique feels a mite old-fashioned. But it enables the kind of civilized ''Professor Plum did it with the lead pipe in the conservatory'' guesswork that never really goes out of style.