Dancers are young when they first dream of dance. Siena was six -- and her dreams kept skipping and leaping, circling and spinning, from airy runs along a beach near her home in Puerto Rico, to dance class in Boston, to her debut performance on stage with the New York City Ballet. <P> <I>To Dance</i> tells <I>and shows</i> the fullness of her dreams and her rhapsodic life they led to. Part family history, part backstage drama, here is an original, firsthand book about a young dancer's beginnings -- and beyond.