Nothing is more ephemeral than the reputations of stage actors -- the truth about their talent dies with the last person ever to have seen them on stage; they leave no record but their myth. So it may be less important to know whether Alfred Lunt and Lynn Fontanne were actually great actors than to understand that, from the 1920s through the 1950s, they were thought to be the greatest of them all. As a showbiz couple, one might call them typical -- she lied about her age, he may well have been gay, and they had fabulous friends. As a durable phenomenon, they were extraordinary -- decades after close friend Noel Coward wrote them the play that gives this book its title, Edward Albee begged them to star in his work. As innovators, they virtually invented the overlapping-dialogue style that inspired dozens of '30s comedies. And as performers? Give Peters credit for infusing this enormously entertaining book with a genuine, specific sense that they weren't to be missed, if only because, in the immortal assessment of Holden Caulfield, ''They didn't act like people and they didn't act like actors.''