Cosima Wagner was one of the most intelligent, passionate and strong-willed women of her time -- and it was a long time, 1837 to 1930, beginning with the coronation of Queen Victoria in England and ending on the brink of Hitler's rise to power in Germany. Cosima's ideals overlapped those of both these figures. She was one of history's most primly Victorian adulteresses, while she was bearing Wagnerian children out of wedlock -- or rather in wedlock, but not to Richard Wagner.

Her mind, in tandem with that of Wagner, her second husband, helped prepare the world for Hitler with an explosive mixture of anti-Semitism (though some of her best friends were Jews) and heroic Teutonic mysticism (though she was born in Italy of French and Hungarian parents). She was the inspiration and a key assistant in the production of some of the 19th century's most memorable music. And she was a driving force in the establishment of the unique institution at Bayreuth, the international temple of the Wagner cult, where she served as the high priestess for nearly 50 years after his death.

To Richard Wagner, Cosima was a collage of all his heroines. "You are Elisabeth, Elsa, Isolde, Brunnhilde and Eva, all in one," he told her. She has been no less fascinating, though perhaps not quite such an object of adoration, to later men who have studied her life. Critic George Steiner, after reading her diaries, found her "awful and magnificent," and to historian Peter Gay she was "exasperating . . . unlovable . . . interesting." When she died, music critic Olin Downes said that she was "known less than any of the other dramatic figures of the Wagnerian past" and would probably remain "the most enigmatic and by no means the least significant of them all."

Since her death, Cosima Wagner has become less enigmatic -- at least in the sense that documentation of her life exists by the truckload, including pounds and pounds of her own diaries, where she serves as a secondary figure to the godlike hero she loved and ultimately married. All her life, she was surrounded by prolific writers of letters and diaries, and though efforts were made to destroy or alter some of the more sensitive documents, there enough -- to wipe out most enigmas.

Despite the obvious strength of her own personality, Cosima Wagner spent her life as a satellite to the even more forceful and talented people who surrounded her. Even after Richard Wagner's deaths, she existed not so much as an individual in her own right by as chief custodian of his heritage. She lived in a time when women were supposed to be submissive, and she had been conditioned from birth to stand in awe of those near her. Her role, she felt in both her marriages, was not to excel personally, but in the words of her biographer, George R. Marek, to "inspire, spur on, prompt, encourage . . . to take part in the shaping of a work of art."

Her first husband, Hans von Bulow, was one of the most gifted performing musicians of his time, but not much of a creator. A Prussian of the old school, he apparently sometimes beat his wife and certainly kept her in an atmosphere she found stifling. But they had at least one thing in common; both were hero-worshippers, and the hero they jointly worshipped was Richard Wagner. In a sense, she tried to turn Bulow into a second Wagner, and left him after it became obvious that he was not made of the right material. He became the world's leading Wagner conductor and remained so after he had lost his wife to his idol. She became Wagner's comfort, refuge and inspiration. But while she was transforming herself to fit his needs, she also mastered the art of dominance through submission, the final resort of strong, intelligent women in a rigid patriarchy. She "opened all letters addressed to Wagner," even those from his former wife, Minna, "and suggested the answers." She chose his friends, manipulated his attitudes and influenced his artistic production.

Cosima had learned from birth how to deal with strong, temperamental personalities. She was the second illegitimate daughter of Franz Liszt and Marie Catherine Sophie de Flavigny, Countess dhAgoult -- not only a member of the nobility by a famous beauty. Even among the children in this small, impermanent menage, she took second place; her older sister, Blandine, was "the pretty one," and she was "the brainy one," nicknamed "the stork" because she was so tall and thin (and also, perhaps, because of her prominent nose). Neither of her parents was qualified for the job, even during the few years when they were in love. And after the passion cooled, the children became a sort of rope in a tug-or-war between the parents. c

"Be convinced," Franz Liszt wrote to his daughters "that it was never my intention to use you as a pretext for a quarrel with your mother. If up to now I deemed your separation from her as inevitable, and insisted on it, I was prompted solely by a high concept of my duty ."

Clearly, he was using his children precisely as a pretext for a quarrel. As for his paternal duty, he began to shirk it as soon as he became a parent. The first time Cosima saw Richard Wagner, she was not quite 16 and he was in the company of her father, whom she had not seen for eight years -- more than half her lifetime.

Besides the excitement of midwifing masterpieces, she seems to have longed for stability and tranquility through her early life, and whatever his shortcomings, Wagner managed to provide her that. He also gave her a kind of recognition, a validation, that had eluded her until he came along. Ridiculous and sometimes unpleasant as these two people may seem, theirs is a true love story -- all the more touching, perhaps, for its absurdities -- which George Marek explores delicately in his biography.

Marek is a well-seasoned musical biographer, with works on Beethoven, Chopin, Mendelssohn and Toscanini among his credits in the field. His latest effort matches the high standard established in earlier works; he is thorought and solid, but does not let scholarship deaden his style and finds room for human interest. If he cannot exactly make his subject seem lovable, he does explain and justify the interest that has clung to her name.