When he was 17 years old, Steve Reich bought a book by William Carlos Williams and discovered a poetry that would affect him for decades. ''I picked it up just because his name read the same forward and backward,'' the composer recalls with a smile. ''But he's the poet I've been closest to all my life.''
Now that enthusiasm has borne fruit in a new Reich composition that borrows its text from Williams's verse. Scored for 89 instruments and 17 voices, ''The Desert Music'' is on a larger scale than any Reich piece to date, while retaining such ''minimalist'' trademarks as pulsing rhythms and repetitive phrases. The composer sees it as a homage to Williams, stressing the poet's awareness of global dangers in the 20th century. But mostly he sees it as an adventure in musical language.
''The Desert Music'' will have its American premiere this Thursday at the Brooklyn Academy of Music in the celebrated ''Next Wave'' festival, with Michael Tilson Thomas conducting. After two more performances on Oct. 28 and 29, it will be recorded for a Phillips release next year. Future performances will be given in London by the BBC Orchestra next July as part of the Proms Concerts, and at the Festival d'Automne in Paris next autumn. The Danish Orchestra and Chorus will give it a Copenhagen premiere in early 1986. It was first heard in Cologne, West Germany, last March.
One section of the ''Desert Music'' text includes Williams's suggestion that ''man has survived hitherto because he was too ignorant to know how to realize his wishes. Now that he can realize them, he must either change them or perish.'' Discussing the piece in his Manhattan digs the other day, Reich acknowledged that these words are ''loaded'' with meaning and urgency.
But he didn't shy away from including them in the piece, because he wanted to confront two questions facing him as a composer: What are the implications of using words in a musical work, and what is the nature of a ''political'' piece of music - if there is such a thing in the first place?
''Williams wrote this in 1954,'' says Reich. ''That was fallout-shelter time. It was after the (nuclear) bombs had been dropped, and he was acutely aware of that.'' Reich considered this context important. He didn't aim to write ''a political piece in the sense of Weill or some things by Henze,'' but in his view ''the words do mean something in a piece of music'' - although, he quickly adds, ''It's the musical setting that does or doesn't engage us. If the music doesn't engage us, we don't give a hoot about what the words are saying.''
Reich also had a context of his own to consider. ''I was writing the piece around Thanksgiving of 1983,'' he explains. ''That was the high point of the Pershing missiles in Europe being set up. Newsweek had a cover on the end of the world and ABC had a documentary mock-up of how America could be destroyed. I remember waking up with a cold feeling that something really wrong was going on, and that my music was going to end up as radioactive pieces of paper floating around what used to be lower Manhattan.''
His mood affected his work. ''I have a reputation as sort of an abstract, hard-line, structuralist guy,'' he notes ironically. ''People may wonder what I'm doing with my foot in something that has meaning!'' But he had entered political territory before, with an electronic piece called ''Come Out'' that uses the voice of a black man in a tense civil rights situation. And he feels his contribution in that work was specifically musical. ''If that was a successful piece,'' he says, ''it wasn't because I was defending civil rights at a time when that was a hot issue. It was because something happened in the tape work itself, and therefore you could take notice of - and think about - what was going on.''
In composing ''The Desert Music'' he also had Williams as an inspiration. ''The tone isn't a preachment,'' he says, ''although Williams was a moralist. It's the voice of a fellow sufferer. The texts are very direct, but their assemblage has an ambiguity that gives them depth and resonance and doesn't make things simplistic.''
This ambiguity, he adds, has special importance in the last movement of the piece. ''The ending has a harmonic ambiguity because there's no room for a hallelujah finale,'' he explains. ''We don't know the ending of these problems in the world we live in. It would be vulgar to tack down the ending, and defeatist to have a negative finish. It has to be ambiguous if you're going to be faithful to the facts....''
Reich is best known as an originator and proponent of the style called ''minimalism'' - a term that has lost whatever literal meaning it once had as music by Reich and Philip Glass, among others, becomes ever more lavish and complex. He is proud of his music's evolution, and he has ideas about why it attracts such large and diverse audiences.
''The bedrock of its popularity is two musical facts - that it has a marked, regular beat and a very clear tonality,'' he states. ''And also, the repetition of motives is understood by everyone in every culture, because it appears everywhere in one way or another.''
In the same vein, Reich says, ''This is the first classical music in a long time that has a kissing-cousin relationship with rock-and-roll and jazz. That has been clasped to the bosom of thousands of listeners, and I'm delighted.
''It was the same case with J. S. Bach and everyone in the baroque period,'' he adds. ''People who danced to galliards and sarabands knew what was going on when they heard them in violin sonatas. They didn't wonder what was happening, as people do if they come from a disco into an Elliott Carter or a Schoenberg concert. That doesn't make Schoenberg bad. Like many others in the history of Western music he will have a special, recherche, refined place where the few will always remember his works. But no postman will ever whistle his tunes, as he hoped they would.''
Despite his own love for the subtleties and complexities of Schoenberg and some other European composers, Reich takes a populist view of American music. ''The tie between the man in the street and the man in the concert hall must be firm,'' he says, ''or there's something wrong with the concert hall. Years ago people said: 'If you don't understand my work, get a degree in music.' How ridiculous! If you don't understand the music, there's something wrong with the composer. Or it's a very special case - and I don't think the special cases were in this country.
''The great American music has been comprehensible,'' he continues with gusto. ''A wide spectrum of people enjoy Ives. A very wide spectrum enjoy George Gershwin and Aaron Copland. And those are our greatest composers! Not difficult, crabby types, but men with the common touch. The same goes for our great poets: Walt Whitman and William Carlos Williams, and even Ezra Pound, though he was weird....''
Discussing his own music, Reich says he has ''fewer and fewer credos'' as he goes along. He describes his early musical ideas as belonging to ''a young man who had discovered something and was single-mindedly, maniacally pursuing it.''
Now he feels they have become so established that ''the humanly and artistically interesting thing to do is develop them, thicken the plot. I have always been an intensely emotional person, and I think the emotional range is wider in my recent pieces.''
As for minimalism at large, he has a happy progress report. ''What was yesterday's avant-garde becomes tomorrow's academia,'' he smiles. ''The universities are producing this music now. I think it is going to become a new academic pursuit. And I think it's had an enormous influence on American music.''
Other offerings from Reich in the near future will include an Angel recording of his Octet in a chamber-orchestra version called ''Eight Lines,'' and early next year a record (on either Phillips or Nonesuch) of the San Francisco Symphony playing his Variations for Winds, Strings, and Keyboards.
He and his own ensemble, Steve Reich and Musicians, will perform his brilliant ''Drumming'' on Dec. 12 at the Museum of Modern Art in New York, as part of an exhibition of ''Primitivism in 20th-Century Art.'' He also has a number of commissions for new pieces from such major institutions as the San Francisco and St. Louis Symphonies, and he will soon collaborate with choreographer Laura Dean for the first time in many years.
Reich seems aware that minimalism has become trendy but takes a long view of his own place in the annals of music. ''Movements come and go,'' he says. ''And when they go, the people who climbed on the bandwagon find it's out of gas. But the principals are left doing what they always did - because they're not on a bandwagon, they're doing what comes naturally. I believe there is a staying power to some of this music, and that it will survive.''
In any case, he says, more and more musicians are learning the skills needed for performing work in his style. ''From the most reserved orchestras to the most adventurous 20-year-old chamber groups,'' he declares, ''this music is getting the opportunity to be played and played well....''