AFTER ALL the memories and biographies, monographs and coffee table ornaments that have been published about the celebrated dancer Vaslav Nijinsky and the revolutionary Ballets Russes de Serge Diaghilev, it doesn't seem possible anything new could be added to this important ballet iconography. But Bronislava Nijinska's book is a revelation on several counts besides its evocative, lively readability.

In the early part of this satisfying book, Nijinsky's sister tells of a childhood moving about Russia with their peripatetic dancer-parents, the closeness of the three talented children (Stanislav, the oldest, entered a psychiatric hospital as a teen-ager and died there in 1918), their lessons and holidays, their father's desertion and their mother's efforts to maintain the family in St. Petersburg. Vaslav and Bronia, two years apart, attended the Imperial Theater school in preparation for their dancing careers.

A good academic student as well as a dancer, Bronia began writing the lifelong diaries and notes that formed the basis for the present book when she was still a schoolgirl. Although she redrafted this material into book form at the end of her life--she died in 1972--the English version by her daughter and Jean Rawlinson retains the immediacy of a firsthand account.

The Vaslav of this narrative emerges as a normal, mischievous and inquisitive boy, not the withdrawn primitive he appeared during the years of his fame, from the first Diaghilev season in Paris, 1909, till he was overtaken by schizophrenia during World War I. Unlike those familial apologias designed to right all wrongs and assure the immortality of famous persons, this book is convincing in artistic matters, reticent in personal ones. Bronia describes how Vaslav worked on his early roles and choreography, developing an original artistic identity along with his prodigious technical gifts. When he joins Diaghilev, he seems to grow distant and impersonal to her, he becomes the great star Nijinsky--but not the cringing puppet others have pictured.

In wonderfully unromantic but informative language, Bronia details the first dramatic seasons of the Ballets Russes, where she too was emerging as a notable dancer. Matters of conjecture in the transitory, often wildly subjective lore of ballet are illuminated, like Nijinsky's entrance as the Golden Slave in Scheherazade: "His bare torso twisted in the fervor and excitement of his newfound freedom, like a cobra about to strike." Ballet lovers and ballet dancers should treasure the precision of what she says about things like the Slave's death throes: "His legs shoot upwards; he stands on his head and rotates his lifeless body--Nijinsky made a full pirouette standing on his head--before dropping to the ground with a heavy thud." Or her teacherly explanation of how he strengthened his feet and how he used them to give the illusion that he was jumping without preparation and didn't have to touch the ground between flights.

What is so striking about these accounts, and about the book as a whole, is the intensity and clarity of Nijinska's focus. Events are real to her precisely as they happen; nothing more. For instance, she describes the delays and difficulties connected with Michel Fokine's Daphnis et Chloe (1912) as an element in the growing rivalry between the choreographer and Vaslav, who was creating his first ballet L'AprMes-Midi d'une Faune at the same time. But there's very little information about Daphnis itself, and no hint of its reception when it finally was premiered. Wisely, I think, Nijinska left the follow-ups and the contexts to history. Similarly, she is more attentive to the dancers' interpretations of roles than to the larger structures of choreography, which are better known to the public anyway.

Through the book runs a sort of fatalism, an acceptance of instantaneous fame or ruin, of the capriciousness of the public and the instability of alliances among artists, whether under the opulent patronage of the czars or the tightrope survival methods of a brilliant independent like Diaghilev. The book ends abruptly in 1914, after a a disastrous but little-reported season Nijinsky gave in London with a pickup company following his dismissal by Diaghilev. When the brother and sister met again after the war, Nijinsky was irrevocably mad. This tragedy seems neither more nor less incredible than the events which have preceeded it.

Nijinska planned another volume, now being completed by Irina, Rawlinson and Holt, Rinehart. I expect it will tell us more about Nijinska's distinguished choreographing career, a body of achievement always unjustly overshadowed by the legend of her brother. The choreographer of Les Noces says she was inspired by her brother's vision, but she too was a genius, something the world doesn't yet know.By MARCIA B. SEIGEL; MARCIA B. SIEGEL, dance critic for New York's Soho Weekly News and The Hudson Review, is the author of The Shapes of Change, a study of American choreography.