Stevens was born in 1879 in Reading, Pennsylvania, the second of five children. His father, from humble beginnings, was a successful lawyer, his mother a former schoolteacher. Each night, she read a chapter of the Bible to the children, who attended schools attached to both Presbyterian and Lutheran churches, where the music left an indelible impression on Stevens. Both sides of the family were Pennsylvania Dutch, an identity that meant little to him when he was young but a great deal later on, perhaps to shore up a precarious sense of identity. (He became obsessed with tracing his family genealogies, poring over thousands of documents, and was “deeply disappointed,” Mariani writes, at being denied membership in the Holland Society of New York when, in the poet’s words, “some bastard from Danzig” popped up to spoil the requisite ancestral purity.) His father, a stern man, urged upon him a regimen of “work and study, study and work,” toward a professional career. Stevens was often ill, to the extent that he had to repeat a year of high school, and a bout of malaria—as improbable as that sounds, in Pennsylvania—permanently impaired his hearing. But he played football, consorted with the town’s bad boys, and cultivated a blustery front.

He also had a hunger for erudition, expressed in precocious poems, essays, and orations. In 1897, he enrolled at Harvard, where he studied closely with the humanist philosopher George Santayana, debating matters of belief (Stevens was afire with skepticism, against Santayana’s more nuanced views) and even exchanging sonnets on the subject. He became the editor of the Harvard Advocate, read widely and deeply, and mastered French on the way to commanding a fabulous vocabulary, choreographing such tangos of words regular and rare as “The squirming facts exceed the squamous mind,” in “The Connoisseur of Chaos.” On graduation, in 1900, he moved to New York and wrote for newspapers. For one, he covered the second Presidential campaign of William Jennings Bryan, whom he hopped home to Reading to vote for. In his third book, “Owl’s Clover,” issued by a leftist publisher, in 1936, Stevens made haplessly clumsy allusions to social and political tensions of the time, though he was “a Hoover Republican,” Mariani writes, and also an admirer of Mussolini for rather longer than is comfortably excused as a common myopia of the time. He was no better than most white men of his class in point of casual racism and anti-Semitism, though fewer such toxins leak into his poetry than into that of Eliot or Pound. In verse, Stevens transcended anything mean or petty in himself, but for art’s sake; he wasn’t much given to moral scruple.

For the New York Tribune, in 1900, Stevens covered the funeral of Stephen Crane, whom he admired but whose mourners he found “wretched, rag, tag, and bobtail.” He thrilled to a performance, in French, by Sarah Bernhardt, as Hamlet, for what he later recalled as her “intricate metamorphosis of thoughts”—quite the keynote of his own developing sensibility. He was bemused by the “quick, unaccountable” life of the city, and took to sitting for spells of restorative peace in St. Patrick’s Cathedral—unbelieving, but savoring the aura of sanctity. Tiring of journalism and seeing no path to a life in literature, he succumbed to pressure from his father and enrolled in the New York Law School. He passed the bar in 1904 and worked at various law and insurance firms.

Also in that year, Stevens fell wildly in love with Elsie Kachel, a Reading girl from a family who lived on “the wrong side of the tracks,” Mariani writes—a cliché now that was at the time a grinding social fate in railway-divided American towns. When his father vehemently opposed the match, Stevens stormed out of the house and never spoke to him again. (He generally avoided all his relatives except, by way of genealogical research, those who were dead.) Elsie was beautiful. In 1916, her profile, sculpted by an artist who was a chance acquaintance, is said to have become the face of the dime, reigning there until she was replaced by F.D.R., in 1946. (Mariani believes the oft-told story, though the artist’s son denied it.) She was also prim, humorless, and, having left school in the ninth grade, intellectually defensive and incurious—traits overlooked by the smitten Stevens through the years of their courtship, while he accrued enough income, by his conventional lights, to justify marriage. The couple wed in 1909 and moved into an apartment on West Twenty-first Street.

The next few years, spent on a small but seething scene of budding modernists, were golden for Stevens’s formation as a poet. At the salon of Walter Arensberg, a wealthy doyen of the new, Stevens met Marcel Duchamp—one of their conversations, in French, suggested to Stevens “sparrows around a pool of water”—and the New Jersey pediatrician and brilliantly innovative poet William Carlos Williams, his peer and cordial rival, who once called him “a troubled man who sings well, somewhat covertly, somewhat overfussily at times, a little stiffly but well.” Williams’s vernacular free verse and Stevens’s sumptuous blank verse long remained magnetic poles of American poetic form. They more or less merged in the work of Marianne Moore, whom both men esteemed.

Mariani’s chapters on these years sparkle with personalities, anecdotes, and ideas. There’s Carl Van Vechten, calling Stevens “a dainty rogue in porcelain” who was “big, blond, and burly”—he stood six feet two—but possessed of “a tiny reserved spirituality.” Arensberg promptly revised the description to “that rogue elephant in porcelain,” in view of Stevens’s social ineptitude. (The patron’s stated formula for a successful poets’ salon was to convene “five or six men who live in the same town and hate each other.”) One gathering was so much fun that Stevens sent a telegram to Elsie, not daring to phone, to say that he would be home late. He admitted to his companions that he dreaded what awaited him at home.

Mariani gives a fascinating account of a poet, previously unknown to me, who strongly influenced Stevens in those days: Donald Evans, a free spirit with a bejewelled, determinedly decadent poetic style, who most probably committed suicide, in 1921. “With their silk-swathed ankles softly kissing,” a typical line reads. Something of Evans—French elegance crossed with American vigor—informs Stevens’s early “Peter Quince at the Clavier,” which weaves theories of music and beauty into a comic version of the story, in the Apocrypha, of Susanna’s harassment by lusting elders: “She turned— / A cymbal crashed, / And roaring horns.” And: “Beauty is momentary in the mind— / The fitful tracing of a portal; / But in the flesh it is immortal.”

Some of Stevens’s breakthrough works amount to literary equivalents of the formally audacious still-lifes and interiors of advanced French painting. The masterpiece “Sunday Morning,” from 1915, is an argument for spirituality without God, interlaced with a woman’s parlor daydream. It begins with “Complacencies of the peignoir, and late / Coffee and oranges in a sunny chair”; ranges “Over the seas, to silent Palestine”; decides that “Death is the mother of beauty; hence from her, / Alone, shall come fulfillment to our dreams / And our desires”; and concludes with a breathtaking image of “casual flocks of pigeons” that, at evening, “make / Ambiguous undulations as they sink, / Downward to darkness, on extended wings.” It was the first poem to appear under Stevens’s name in Poetry: A Magazine of Verse, which had recently started publication in Chicago. (He had shyly used a pseudonym, Peter Parasol, when submitting earlier poems, two of which were accepted.)

The editor, Harriet Monroe, cut some stanzas and rearranged others, and Stevens agreed to it, though he restored the original in “Harmonium.” A certain reciprocal high-handedness among poets and editors—as if the modern in aesthetics required a team effort—marked the time. (Think of Pound’s retooling of “The Waste Land.”) Williams advised Stevens to delete, from a poem, two lines that struck him as sentimental. “For Christ’s sake yield to me and become great and famous,” he hectored. Stevens obeyed.