"Athletes have it easier than opera singers," says Ethan Mordden, "though singers are a form of athlete . . . A baseball player who fails to hit a home run isn't booed out of the stadium. A runner trying and failing to break a record has our sympathy. But a diva who misses a high note is in Big Trouble."

His observation, like his book, focuses on the diva, the larger-than-life soprano who has been the focus of frenzy among opera fandom's lunatic fringe (replacing the castrati who became extinct, and mezzo-sopranos, who went out of style) for about 150 years.

The form of madness he describes is slightly old-fashioned -- more appropriate to the atmosphere in major opera houses a generation ago than today. An audience today looks more at the lighting, costumes and scenery, the stage direction, the total effect. It buys its tickets for a full evening of relatively balanced and integrated entertainment. Occasionally, it will even give its biggest ovation to a baritone who has done an outstanding job, or to a stage director who has cast familiar material in a new light, rather than the Aida who has successfully leaped up to a high C or the Queen of the Night who has made it all the way to F without mishap.

Mordden is aware of these developments and mentions them in passing. But if his viewpoint is partial and specialized, it still has a kind of fascination. It is probably still substantially true in Milan and Vienna, where opera is a sort of religion. As his title makes clear, Mordden is writing about a small, specialized world -- the world of the fanatic. It is not the only world in opera, but it is interesting enough to be worth a book -- specifically, this highly opinionated, anecdote-loaded, obsessively readable book.

"Demented" is compounded of three overlapping ingredients: plagiarism, big-name-studded anecdotes and worldly wise general observations about the hothouse world of the prima donna. The anecdotes are carefully culled and frequently accurate. When an anecdote is more interesting than true, Mordden does not hesitate to tell it in detail and then gleefully destroy it in a footnote. But the plagiarism is the element that gives "Demented" its special flavor.

Mordden makes no secret of his plagiarism; he proclaims it, in fact, with a cheerful energy that might make a thoughtful reader slightly suspicious. A significant part of the book's contents, he confesses, is lifted from a "classic pamphlet," "Der Weltsopranfu hrer" ("The Prima Donna's Handbook") written by Lotte Heinotz, "a curiously unpopular singer of minor roles active in Vienna in the early 1900s" after she had "retired, at the urgent request of the city of Vienna." Heinotz occupies a curious position in "Demented." She is the ugly duckling on a lake full of swans, but her voice can be heard in the calm, analytic, slightly superior commentary that runs throughout the book.

In a book that resounds with the names of dazzling performers -- Maria Callas, Joan Sutherland, Kirsten Flagstad, Renata Tebaldi -- she is the only one of her kind: the one who knew exactly what was needed and simply did not have it. She provides Mordden's basic analytic tools -- for example, the crucial distinction between the "Stimmdiva" (the great voice) and the "Kunstdiva" (the total artist). She also provides some of the best quotes: "The Stimmdiva's idea of preparing Guonod's Marguerite involves learning the music and ordering her outfits. The Kunstdiva reads Goethe."

But more than that, Lotte Heinotz provides that necessary element of shadow in a book that might otherwise have become a nightmare of almost unrelieved glitter. If she did not exist, Ethan Mordden would have been forced to invent her.

Perhaps he did. "Der Weltsopranfu hrer" has been long out of print. I have never seen a copy, nor have any of the opera fanatics I have consulted. Lotte Heinotz is also unmentioned by the scrupulous Nicholas Slonimsky in "Baker's Biographical Dictionary of Musicians," a name found nowhere in the 20 volumes of "The New Grove Dictionary of Music and Musicians." It is a curiously onomatopoetic name, a name that rhymes exactly with "Lotta high notes." In the final analysis, of course, the existence of this towering figure is a matter of secondary importance, like the existence of Oedipus or Tarzan. What matters is the symbolic importance of the figure. Whether or not she has ever existed, Lotte Heinotz lives.