We are here together, ten of us on this small and lonely island. And one of us is a murderer."" Sounds familiar? Of course--because, while never for a moment relinquishing her own rich, dark personal style, James has made her latest mystery something of an hommage to Agatha Christie: the isolated island, the tricky interplay of motives, the rather theatrical cast-of-characters. And, even better, James has brought back one of detection's Great Under-Exposed Favorites--Cordelia Gray of An Unsuitable Job for a Woman--as sleuth. (Commander A. Dalgliesh is talked about here but never appears.) Cordelia, struggling private eye, is hired as weekend bodyguard/companion to actress Clarissa Lisle--a fading great who's been beleaguered by creepy, anonymous, poetic death-threats. The setting: Courcy Is-land--where nervous, egocentric Clarissa, as castle guest of effete, erudite Ambrose Gorringe, is to play the lead in an amateur performance of The Duchess of Malfi. The fellow guests: Clarissa's lovesick spinster-cousin Roma (desperately in need of cash); her fidgety young stepson; her Mrs. Danvers-like dresser; her ex-lover, critic lye Whittington (bitter, terminally ill); and her fourth husband George, a military man with ugly WW II secrets. So there are motives galore when Clarissa is found bashed to death just before the performance. . . soon to be followed by the drowning of weird butler Munter. And, though the solution is a trifle ho-hum (a sliver of psychopathology), the detours along the way are splendidly Christie-ish. Finally, however, James scores not with plot but with texture: the understated humor, the stately yet unpretentious prose, the psychological insights, the quiet charm--plus, above all, the fundamental warmth and wisdom in every line she writes. In short: another James triumph--a bit less ambitious than her others, perhaps, but no less distinctive or absorbing.