An incredible book for the same audience as Joseph H. Mazo's Dance is a Contact Sport (1974), but with less boyish enthusiasm and a very different sort of grasp. What Gruen has done is to assemble scores of interviews with central figures going back to the original Ballet Russe. Some who speak are Rosella Hightower, Dame Margot, Danilova, Markova and a Coward-esque Anton Dolin, Nureyev, Bejart, and Alley -- a whale of a scope, with interests as diverse as Mr. Balanchine's wives and Arthur Mitchell's project in Harlem. The interest in fact seems just about inexhaustible, and the audience we'd guess to be big.