In 1955 Henry Pleasants, a critic of both popular and classical music, issued a cranky screed of a book, “The Agony of Modern Music,” which opened with the implacable verdict that “serious music is a dead art.” Pleasants’s thesis was that the traditional forms of classical music — opera, oratorio, orchestral and chamber music, all constructions of a bygone era — no longer related to the experience of our modern lives. Composers had lost touch with the currents of popular taste, and popular music, with its vitality and its connection to the spirit of the times, had dethroned the classics. Absent the mass appeal enjoyed by past masters like Beethoven, Verdi, Wagner and Tchaikovsky, modern composers had retreated into obscurantism, condemned to a futile search for novelty amid the detritus of a tradition that was, like overworked soil, exhausted and fallow. One could still love classical music, but only with the awareness that it was a relic of the past and in no way representative of our contemporary experience.

While Pleasants’s signaling the ascendance of popular music was right, much of the rest of “The Agony of Modern Music” was fallacious, not least its way of according value to a work of art based on the size of its audience. The book went out of print, deservedly so, but its title still lingers on as an uncomfortable meme expressing a collective anxiety about the direction that classical music has taken over the last hundred or more years. And for a large part of its public, no composer is more emblematic of that persistent feeling of alienation between composer and listener than Arnold Schoenberg.

That is a situation Harvey Sachs hopes to change in his book “Schoenberg: Why He Matters.” Sachs for decades has written mostly on conventional classical music subjects with titles such as “Ten Masterpieces of Music,” “Virtuoso,” “The Ninth: Beethoven and the World in 1824,” and three books on Toscanini. He is a sterling example of the good old-fashioned and sadly vanishing “music appreciation” writer, albeit with a sophisticated command of the historical and political back story. For Sachs at age 77 to produce this impassioned defense of Schoenberg, composer of some of the most difficult and intimidating music ever written, might seem surprising, but the totality of Schoenberg’s life — as composer, painter, writer, teacher, exiled Jew and profoundly influential thinker — comprises one of the great narratives of 20th-century Western culture, and one can see how the story of this artist’s struggle for acceptance against the backdrop of the societal calamities of his era was so appealing to Sachs.

Schoenberg came of age during the apogee of Viennese culture, the feverishly productive period of social and artistic activity that spanned the years from 1890 to the onset of the Great War, a time we associate with names like Mahler, Klimt, Freud, Hofmannsthal, Max Reinhardt, Stefan Zweig and … Adolf Hitler. Vienna was a notoriously antisemitic city, and Schoenberg, like his supporter Mahler, performed a necessary balancing act between his love for its musical past and coping with the stress of humiliating discrimination. His intellect was all-inclusive. He exhibited a lifelong truculence toward any and all conventions that he himself had not examined firsthand. His creative impulse was so overflowing that at times composing was not enough. During his 30s he took up painting very seriously. He wrote a play proposing a “new Palestine,” crafted his own librettos, learned bookbinding, and in his later California years studied tennis with the same analytical precision that he brought to his music. As a teacher — he claimed toward the end of his life to have taught over 1,000 students — he exerted an influence that lasted for decades, even after his death.

His first mature pieces were in a moody, emotionally turbulent post-Wagnerian style. “Transfigured Night,” “Pelleas und Melisande” and “Gurrelieder,” all written around the turn of the century, are tonal and share many traits with Mahler and Richard Strauss: extended musical forms, restless, roving harmonies, explosive climaxes and a rich, prodigious use of the orchestra. “Gurrelieder,” a 90-minute oratorio-like work for a gigantic conglomeration of orchestra, choruses and solo voices, is the ne plus ultra of extravagant last-gasp Germanic Romanticism.

But even when he was writing in the tonal idiom of these early works, Schoenberg never made it easy for his listeners. He was, as Sachs points out, “difficult” from the very start. His first string quartet goes for a full 40 minutes of frenetic energy entirely without pause. While Strauss would annually deliver a tone poem or opera that became an instant hit, Schoenberg’s music remained respected but rarely performed.

And then suddenly he executes a volte-face, one of the most shocking stylistic changes in the history of classical music, trading the lavish, hypertrophied forms of the early works for a new language of compressed, often gnomic utterance. Tonal relationships begin to implode; familiar formal templates disappear; and the emotional ambience, especially in theatrical works like “Pierrot Lunaire” and “Erwartung,” becomes eerie, inward, ghoulish, even psychotic.

Clichés abound in describing what happened to tonal harmony in the works of this period (and of those of his two famous students Alban Berg and Anton Webern), the most persistent of which is that tonality was “exhausted,” or “collapsed,” i.e., that by 1910 everything that could be discovered about harmony had been found and exploited and that there was nowhere to go except to abandon it. Schoenberg, by way of explanation, offered the notion of “emancipation of the dissonance,” an optimistic phrase if there ever was one. But the gap between willing listener and iconoclastic composer just grew wider, became a chasm. Without tonal harmony to unite and give direction to the flow of sounds, the listener was more often than not unable to find coherence and meaning in the music.

Sachs describes how Schoenberg, aware of the potentially chaotic direction these expressionistic, free-form atonal works were heading, finally found a way to impose order on their elements: the 12-tone technique, a rigorously exact method of composing that consciously avoided the gravitational sensation of “home key” common to almost all other music. Tonic and dominant, the fundamental rhetorical basis of the musical experience was thereby denied in favor of a sort of democracy of pitches.

Schoenberg’s artistic crisis was playing out against the looming threat of the Holocaust. Although he was 10 years ahead of the curve in identifying Hitler as a threat, by 1933 he was forced to flee with his wife and infant daughter, first to Boston and eventually, at the age of 60, to Los Angeles, where he remained until his death in 1951 at 76. Proud and combative in matters concerning his standing as a composer, he was by all accounts a congenial and loving family man. The Vienna-born composer who was mentored by Gustav Mahler ended up living on the same Brentwood street as Shirley Temple, was friends with Charlie Chaplin and had his portrait painted by George Gershwin. He was even invited to present the Academy Award for best musical score of 1937 but had to withdraw because of illness.

Sachs’ book, despite its urgently prescriptive title, “Schoenberg: Why He Matters” (“Your Lipids: Why They Matter”), is nonetheless an immensely valuable source for anyone desiring an accessible overview of this endlessly controversial and chronically misunderstood giant of 20th-century music. Too many books about Schoenberg are overly technical for the general reader, or else they assume a kind of hagiographic defensive crouch. Sachs can be refreshingly candid, sharing his feelings at times as if he were whispering confidentially in your ear during a concert intermission. He finds some of the pieces borderline hysterical or even ridiculous, and he is frank about admitting the ambivalence that many, this writer included, feel when encountering the thornier of the composer’s works. But his genuine enthusiasm for those pieces that do stir him is enough to draw the reader in, and in so doing has done a great service to the cause. Particularly meaningful is Sachs’ description of Schoenberg’s spiritual life, his re-embracing of Judaism (after an early conversion to Lutheranism) and the epochal struggle to complete his monumental opera “Moses und Aron,” a work that speaks of the tension between the unutterable word of God and our very human need for words and images to confirm our lived experience.