"We are now enjoying the happiest time of our lives," Richard Wagner told his wife, Cosima, near the beginning of this epic volume, which covers the last five years of his life. He was probably right.

The composer's unique shrine at Bayreuth was still financially insecure, but it was established and functioning, and attracting both the interest and the generosity of a society of patrons. Poor, mad King Ludwig of Bavaria, who had squandered a fortune chasing his own dreams and Wagner's, could no longer give him money outright but was arranging a convenient loan. Royalties were coming in from his earlier works. "Yesterday afternoon," Cosima wrote in January 1879, "he came to tell me that the royalties from Vienna have been good -- all his works, even Rienzi, had earned something. I: 'So they are good children, after all.' R.: 'Yes, they have to go begging for me.'"

The epic work of composing the Ring cycle had been concluded, and the even more daunting task of making the world want to hear it was well underway. Wagner's health was deteriorating -- almost every day, in her diary, Cosima reports on his condition and how well he slept the night before -- but he was busily and for the most part happily at work on Parsifal, reading Dante, dreaming of future projects and fantasizing about moving to the United States. He was accepted as a sort of prophet by more and more disciples -- though some of his prophecies have failed the test of time: "Recently, when the phonograph was mentioned, he spoke of the foolishness of expecting anything from such inventions."

Amid the Wagner happiness, there were shadows; the conversation about royalties modulated into a foretaste of death: "Over coffee he says he can only bear his state of well-being when composing -- otherwise it seems to him like a sin. He looks into the salon and says, 'This is where I want to be carried when I die.' I reproach him for using the singular." Four year later, he died -- in the singular. Cosima closed the diary before recording that event and never wrote another entry in it, although she lived on until 1930, keeping and nourishing the sacred flame.

Richard Wagner (who is called "R." in the diaries) may have been the creative genius in the family, but Cosima was a more dedicated diarist, setting down at least a few lines and often a long entry practically every day of their married life. Wagner's own diary, the recently published "Brown Book," begins promisingly enough because he wrote it while he was separated from Cosima, for her to read later. But once they were no longer separated, it was abandoned as a diary and became a miscellaneous workbook in which he would jot down ideas on any writing or composing project that cought his fancy. Its value for Wagner scholarship is considerable -- but after the early entries, the interest is not primarily biographical.

No matter; Cosima's diaries provide all the biographical material anyone is likely to want. There are no startling revelations -- partly because the diaries were occasionally entrusted to Wagner scholars during the century they remained a mystery to the general public -- and there is a serious lack of objectivity and scholarly perspective. But there is a loving abundance of detail. What was Wagner doing exactly a century ago? The entry for Tuesday, Jan. 4, 1881 informs us that it was "A good night for R. and for me -- lovely weather." He took an afternoon stroll, though he had trouble putting on his shirt; admired an artist's sketch of one of his operatic heroines but thought it should be nude, conversed about Vienna, critcized the "frighteningly Slavonic" bone structure of a friend's head, and read aloud for a while in the evening. The table-talk that day was not particularly inspired, but there are many more days of chit-chat, miles upon miles of strolling, evening after evening of private musical entertainments, usually with music of The Master.

One fact emerges from the diaries with striking clarity and gives them their chief value: Cosima was hopelessly, uncritically in love with her husband -- an accomplishment that those who read scrupulously through these pages may find hard to understand. Wagner shares her infatuation, as shown in one curious exchange which she set down for posterity: "When he goes, I call out after him, 'I think only of you! He: 'And I only of myself, for that means you.'"

Whatever element of self-love may be mingled in Wagner's feeling for his wife, however, the mutual warmth of this aging couple pervades the enormous bulk of the diaries. They loved one another literally to distraction: "R. is sitting at his writing desk, and he calls out to me: 'Do you know why I am so absent-minded? Because I think too much about you.' And to this he add such words that, back in my room, I beseech the God who has allotted all this to me to grant me humility!"

It is precisely because her love is so uncritical that Cosima is able to set down such a cruelly detailed and accurate portrait. She shows her subject, warts and all, under the illusion that the warts are beauty marks. Sometimes, even eggs him on:

"I tell him I am always asking myself why the writings of women, whether in prose or in verse, are so curiously dry. R. replies, 'Because there is something artificial about a woman who writes; it is her nature either to love or to hate, whereas for a man writing is a natural means of expression.'"

They even plan to collaborate on the desecration of an idol: "After lunch I beg R. to mark in the scores of Beeth.'s symphonies all the things he would like to see altered. He says, 'After Parsifal.'"

If he could dream of improving Beethoven, that was a compliment; Beethoven was a man he could "worship as a god." Of Berlioz, he says that "It would be better if such music had never been written. Barrenness, touches of vulgarity amid great eccentricity, overattention to detail, yet some true inspiration: "There are themes of his I remember,' R. says. 'One of them in the Adagio of Romeo is wonderful.' 'But he belongs entirely to the French school.' Taken on the whole, a very unpleasing impression."

Perhaps the most unpleasing impression in the diaries is that of anti-Semitism -- a pervasive sentiment, mingled with Teutonic racial mysticism, that Cosima shared eagerly with her spouse. In a post-Hitler world, this material has acquired overtones undreamed when it was set down a century ago -- but Hitler without Wagner in his background would have been a very different phenomenon: "At supper yesterday he talked about an article in the Illustrirte Zeitung, 'The Elk Fighting the Wolves,' and said it had taught him some very curious things -- how in Nature even the most heroic must perish, men as well as animals, 'and what remain are the rats and mice -- the Jews.'"

The musical and ethnic prejudices reinforce one another precisely when Wagner's attention turns to the music of Mendelssohn: "He has just been looking through [Mendelssohn's] Saint Paul and is filled with utter disgust for it, calling it Jewish through and through, facile in form, shallow in content: 'To serve us that,' R. exclaims, 'after we have known Mozart, Beethoven and Weber! -- And as if one could ever hope to imitate Bach!'"

Considering the identity of the subject, music occupies a relatively small place in Cosima Wagner's diaries and much of what is said is as valueless as the remarks quoted on Beethoven, Berlioz and Mendelssohn. But these volumes are priceless for those who would understand Wagner's music and the creator's shifting relation to it -- and on his own work he can be more prophetic than he was on the future of the phonograph: "He say he has produced nothing new since Tristan, whereupon I observe that even from a technical point of view Die Meistersinger, Siegfried, and Goetterdaemmerung contain new ideas. But what he meant was that there had been no need for him to write a single note more; he could just have said, 'Do it as I do.' So he thinks!"

Wagner might have been happy to know that as these lines were being set down there were in Vienna a four-year-old boy named Arnold Schoenberg and a teen-ager named Gustav Mahler who would find their points of departure largely in Tristan and take from there into the 20th century. His feeling might have been tempered, however, by the fact that they were both Jews.

Reading Cosima's diaries is remarkably like having breakfast with Richard Wagner, day after day, for years on end -- on the whole, probably an experience best enjoyed in a book, which can be returned to the shelf when one wearies. She was an ardent and an expert chronicler, and her jottings will be a treasure trove for scholars, a rich object of curiosity for idle browsers. They have been edited most scrupulously, with detailed notes to explain the many obscure passages in the text, a handy chronology of Wagner's life with Cosima and 32 pages of index. It is a curious monument, perhaps, but -- like so much of Wagner's own work -- undeniably monumental.