He has admirers who believe Boulez has earned the eminence by the quality of his musical achievement. Others point to his aggressive rhetoric and the clout he has wielded from various positions of power in behalf of his own approach to music, which is formalist, analytical, intellectual, theoretical and, above all, ultra-serious. His 1971-78 stint as music director of the New York Philharmonic won him few friends; Boulez was widely criticized for his choice of repertory and for conductorial interpretations that many found cold and unexpressive.

In 1980, four years into his directorship of the high-tech Institute for Research in the Coordination of Acoustics and Music (IRCAM) in Paris, the French journalist Harry Halbreich called Boulez a ''new Lully,'' comparing the scopeof his influence to the royally-sanctioned dictatorship of the 17th-century composer. At around the same time, the respected Paris-based Greek composer Iannis Xenakis stated flatly that Boulez represented ''something not far from absolute evil.''

Boulez's literary and musical output slackened in the 1960s as his conducting career grew. Nevertheless, say his champions, what little the middle-aged Boulez has produced in the last two decades is consistent, in both quality and philosophy, with what he did before.

SOME ANTI-BOULEZ feeling could be detected in the audience that gathered Feb. 19 in Northwestern University's Pick Staiger Concert Hall. People came to hear the composer speak about the work - the latest version of an in-progress piece titled ''Répons,'' for chamber orchestra, soloists and almost a million dollars' worth of computer-generated sounds - that would be presented in the university's gymnasium the following evening. ''If precision is so important to you, why not simply remove the performer altogether and just do everything with machines?'' one person asked in the question-and-answer session. ''Can you tell us why a listener should actually like your music?'' asked another.

Various other events in the Chicago area week before last - lectures on Boulez, and student concerts of his older music - were folded in with Boulez's talk and performances by IRCAM's Ensemble InterContemporain. It amounted to a little Boulez festival, marking the famous man's first appearances in the United States since 1978, and the Ensemble InterContemporain's debut American tour.

On the whole, Boulez was very well received by the Evanston audience. He seemed completely at

ease, and he was cordial even to his antagonistic questioners.

He said he would never abandon human performers because he values highly the ''irrational'' qualities that only they can bring to music. ''I want from the machine what is specific to the machine,'' he said, ''and from the performer what is specific to the performer. The machines are useful tools, because they broaden the imagination and allow us to organize sounds in new ways. But irrationality, by which I mean subtle things - expressive things - that are only slightly unpredictable, is a gift from God. It is something I always want to keep at the heart of my music.''

AND IN RESPONSE to the query about why listeners should ''like'' his music, he said: ''I don't know, and I don't have to know, because it's the listeners' problem, not mine. There is a great deal of music that I do not like. This does not mean that the music is inferior; it only means that my taste does not correspond to this or that composer's taste. I compose only to satisfy my own taste, and I know that many persons who do not share that taste. Still, it pleases me whenever I discover that someone - for whatever reasons - is able to find a part of himself in my music.''

To judge from the ovation, most of the 900 or so listeners who packed into Patten Gymnasium on the sleety night of Feb. 20 found at least some part of themselves in ''Répons.''

In terms of gesture and texture, the 43-minute piece is quite similar to such vintage Boulez works as the 1952 song cycle ''Le marteau sans maitre,'' the 1955-57 Piano Sonata No. 3 and the 1957-62 ''Pli selon pli'' for soprano and orchestra: Isolated ''points'' of music are superimposed on ''blocks'' that serve either as punctuations or as resting places, and throughout the five sections one hears - and feels - an irregular flow of activity that is both dynamically compelling from moment to moment and utterly logical in its overall design.

BUT THE SOUNDS of ''Répons'' are unlike those found in any earlier work by Boulez, and the manner in which they're used is distinctly different from the approach taken by any other composer known to me. Six soloists (piano, electric organ, harp, cimbalom, vibraphone and xylophone) are spread about the perimeter of the performance area. Their instruments are amplified, but more often than not what emerges from the loudspeakers is instrumental sonority transformed drastically - and almost instantly - by electronic means. IRCAM's 4X computer (designed by the American musician Andrew Gerzso) does its stuff in a mere 25 milliseconds; it's the fastest computer music instrument in the world, and it is said to offer a literally unlimited palette of tone-colors.

The metallic shimmer that has lately become a cliché of computer-generated music seldom penetrates the fabric of ''Répons.'' Rather, Boulez and Gerzso opt for patently dark hues and deep, rich resonances. The sounds dance through the array of loudspeakers in a way that makes them seem animate creatures; their vibrancy and their shapely envelopes constantly remind the listener of their human origins. The title, ''Répons,'' means a response of the liturgical kind. The 4X responds immediately to what is fed into it via microphones; the soloists too respond - sometimes sympathetically, sometimes argumentatively, always somewhat ''irrationally'' - to the more or less rigid and wholly ''rational'' music of the 24-piece ensemble that with the conductor occupies the center of the room.

Ever since he burst on the scene in the aftermath of World War II, Boulez had been regarded by the American musical establishment as an enemy, an outspoken iconoclast whose radical ideas spelled nothing but trouble for the institutions of the art. But here he was, feted by university sponsors, lionized by the press and cheered by audiences who filled to capacity nearly every venue in which ''Répons'' was performed. Less than a decade had elapsed since Boulez returned to his native France after his controversial resignation from the music directorship of the New York Philharmonic. One had to wonder: Could it be that Boulez and all he stood for were finally being accepted here?

One still has to wonder.

His musical language is perhaps more easily understood today than when it was first formulated 40 years ago, yet Boulez's compositions continue to pose enormous challenges for anyone who chooses to grapple with them. And his essays, which deal with far more than just his own music, remain uncautiously quarrelsome vis-a-vis tradition.

Boulez has always had a loyal following among the American musical intelligentsia. If the generally favorable response granted ''Répons'' last year means that the Boulez camp has now grown to include a fair share of the hoi polloi and at least a few of the movers and shakers, then it would seem that the American musical scene is in for some big changes. Depending on one's point of view, that conclusion can be interpreted either as a promise of salvation or a threat of doom. However one looks at it, though, the issue of Boulez warrants attention from persons who take music seriously; for those who would join in the debate, two new books - a collection of essays about Boulez and a collection of essays by him - offer plenty of food for thought.

The heavier reading, on the whole, is to be found in ''Pierre Boulez: A Symposium'' (Eulenberg/Da Capo Press, $25).

Edited by William Glock, this 254-page paperback volume does include one of the most lucid biographies of Boulez ever written, a profile penned by Peter Heyworth for the New Yorker in 1973. There are also brief, and easily digestible, accounts by Glock on Boulez's six-year tenure as chief conductor of the BBC Symphony and by Jonathan Harvey on Boulez's recent activities at the government-supported Institute for Research and Coordination of Acoustics and Music (IRCAM) in Paris.

But the meat of the book is in its probing analyses of various aspects of Boulez's musical output. The centerpiece is a long chapter by Susan Bradshaw that begins with a resume of Boulez's style in general and then examines in detail virtually every work Boulez has produced (excluding ''Répons'') since 1946. Gerald Bennett's chapter deals in similar detail with virtually everything Boulez wrote be fore 1946, and it includes facsimiles of manuscripts of several unpublished works.

An article on Boulez's three piano sonatas is offered by Charles Rosen; Célestin Deliège contributes a study on the relationship between Boulez and the poet Stephane Mallarmé as exemplified in the 1962 composition ''Pli selon pli.'' Deliege's piece, translated from the original French, is an almost inscrutable hermeneutic of the sort that gives a bad name to that fashionable ''interpretive'' branch of philosophy called semiotics.

Elsewhere the writing is clear; the material, however, tends to be highly technical, potentially difficult even for professionals and hardly the sort of thing on which a new-comer to the Boulez controversy would want to cut his teeth.

It might be assumed that a composer whose music prompts such complicated essays as these himself writes complicated essays.

In Boulez's case, quite the opposite is true. Boulez produces music largely in order to satisfy his own inner needs; he produces essays in order to illuminate that music and to address larger issues in the musical world. As an author, he's functioned as polemicist, analyst, eulogist, apologist, historian and - some would argue - poet. But he's never been one to preach to the already converted. Boulez is, above all, a communicator.

The effectiveness with which he communicates, in prose, can be noted throughout ''Orientations: Collected Writings by Pierre Boulez'' (Harvard University Press; $30).

This is in essence a translation, by Jean-Jacques Nattiez, of a 1985 French anthology titled ''Points de repère''; two items in the French edition have been omitted, and three have been added. There are 68 pieces in all. They range chronologically from a letter Boulez wrote to John Cage in 1948 (an explication of the organizational system in Boulez's ''Polyphonie X'' and ''Structures'') to the acceptance speech Boulez gave upon receiving the Siemens Prize in the spring of 1979.

Early in his career, when he was very much the enfant terrible of post-war France, Boulez liked to shoot from the hip; as a result, he still bears the stigma of an anarchist who once advocated the literal destruction of all relics of the cultural past. Browsing through ''Orientations,'' the reader discovers again and again that what Boulez is actually opposed to is the mindless worship of the past at the expense of cultivation of things of the present. The masterpieces of the past should be preserved, Boulez says, but so should the spirit of adventure that brought those masterpieces into existence in the first place.

Boulez's most pointed comments on modern musical life are to be found in a speech titled ''Ou en est-on?'' (''Where Are We Now?'') he made in 1968 in his home-town of Saint-Etienne.

Musical culture is dying, he told his audience, because so many ''sham phenomena of knowledge'' are preserved in concert halls that, because of their interior architecture, cannot be anything more than museums. Regarding the typical patrons of these museums, he said: ''The worst people of all are the semicultivated, who think they are connoisseurs but really know absolutely nothing.''

But he was just as hard on those who espouse new music without exercising their powers of discrimination: ''They pride themselves on attending the concert and, with a few exceptions . . ., their applause is the expression of self-approbation . . . You have only to observe the way [these people judge each other . . . Everyone says: 'Yes, he is really clever; he agrees with me.' ''

Along with more thoughtful, more thought-provoking presentations of old music, Boulez said in 1968, what is needed is a more suitable physical environment for new music. The traditional concert hall, which by its very design fosters an ''attitude of worship,'' is not at all appropriate for modern music that ''demands the intelligent participation of audiences.'' ''The time hascome . . . to look to other methods, other means of investigation and other means of communication,'' Boulez said. ''Only in this way will music be able to progress.''

In 1970, two years after he delivered his Saint-Etienne speech, Boulez became music director of the New York Philharmonic and tried to put his ideas into practice. He failed, on such a grand scale that when he finally quit, in 1977, his departure was seen by many New Yorkers not as an exit but as an exile.

Immediately upon leaving New York, Boulez took up duties as director of the newly opened IRCAM, a facility he had helped plan from its inception seven years earlier. Among other niceties, IRCAM has a concert room - the Espace de Projection - in which not only the seating arrangement but also the acoustic ambiance is flexible. It was for such a room that ''Répons,'' with its half-dozen soloists and half-dozen loudspeakers surrounding the audience, was conceived. And it was in comparable rooms - gymnasiums, armories and the like - that ''Répons'' was presented during last spring's highly successful tour by Boulez and his Paris-based Ensemble InterContemporain.

People paid close attention to ''Répons,'' as much to the music itself as to the ingeniously filled space it occupied. To judge from the question-and-answer session I heard Boulez host at Northwestern University, people also paid attention to the cultural and sociological ideas out of which the composition at least in part grew.

But ''Rituel'' is nonetheless a significant piece, as worthy as any of sharing the program with the ''Ode to Joy'' and the rest of Beethoven's ninth symphony.

In itself, ''Rituel'' is both a gorgeous-sounding thing and a moving tribute to a modernist composer who - for his personality as much as for his music - ranked among Europe's most revered.

The genre to which ''Rituel'' belongs is characterized not by techniques but by an attitude, a still largely European attitude that has not often been sampled by the Powell Hall audience but which Slatkin in recent seasons has seemed more and more willing to explore. It is an attitude that places little value on flash and dazzle, on impressionistic descriptiveness, on visceral excitement. Rather, it emphasizes ideas, and it invites not just aural but intellectual and spiritual involvement.

In music of this sort, the sonorities and harmonies and melodic fragments, no matter how lovely or catchy, are in the long run mere raw materials and surface touches. The essence of the music is its structure, and it is the way in which that structure unfolds - delicately, or explosively, or, in the case of ''Rituel,'' solemnly and deliberately - that speaks the music's message.

For many listeners, it may be that the most memorable detail of ''Rituel'' will be its delicious vapor of Oriental gongs; for listeners able to use the gong sound as a stepping-stone, as a means of entering into the ''stream'' of the music, surely what will be longest remembered will be the inexorable momentum of its gentle current. One is tempted to hear this as metaphor, to grant extra-musical meaning to a purely musical phenomenon.

''Rituel'' runs deep; like most of what Boulez has produced - and like so much of the music by Maderna, Karlheinz Stockhausen, Luigi Nono, Luciano Berio, Iannis Xenakis, Gyorgy Ligeti and, in this country, Elliott Carter - it is profoundly serious stuff.

Beethoven's ''Symphony No. 9'' is profoundly serious, too, and the performance on Friday left Powell Hall resonating with all its many implications. The interpretation was tighter, and much more successful, than that given the Boulez piece. ''Rituel'' affords a conductor many options regarding the lengths of its various sections, and those taken by Slatkin seemed to have the music bogging down just after the midpoint of its 30-minute course. The score for the ''Symphony No. 9'' is all very specific, and Slatkin's reading of it was for the most part as literal as it was powerful.

The bass soloist, whose stentorian call for a ''sweeter, more joyous music'' initiates the famous ''Ode to Joy'' movement, was Michael Devlin, and he sounded generally unfocused in both pitch and diction. But his colleagues - soprano Deborah Voigt, mezzo-soprano Lucille Beer and tenor Walter Plante - formed an excellent three-fourths of a quartet. Thomas Peck's St. Louis Symphony Chorus, as usual, sang magnificently, and the orchestra played like a world-class ensemble.