Who was Max Jacob? That’s the question poet and University of Chicago professor Rosanna Warren circles around in this detailed, nearly 600-page biography, a project she has worked on for more than 30 years.

Born in Quimper, Brittany, in 1876, Jacob could never settle on, or settle for, a single identity — being a Jew, “cubist” poet, homosexual, painter, punster, experimental novelist, art critic, mimic, professional palm-reader, ether-addict, Kabbalist, Catholic convert, prolific letter-writer, mentor to the young and, for many years, the best friend of a Spanish artist named Pablo Picasso.

Like his English contemporary Max Beerbohm, the French Max was also a dandy, one who sported a monocle, favored pink shirts and carried a walking stick. This doesn’t mean he was well-to-do: In his younger days he would hawk his poetry collections in cafes to earn a few francs. A party animal and irrepressible flirt, this Harlequin of Montmartre was nonetheless deeply drawn to the mystical and spiritual, twice experiencing visions of Jesus Christ, the second time in a crowded cinema. Still, as Warren emphasizes, even a deeply devout Catholic — who regarded the Host as a kind of aspirin for every spiritual malaise — was still a Jew in an often highly anti-Semitic France. In 1944, Jacob was consequently imprisoned at Drancy, the French holding center for Jews destined for transport to Auschwitz. Two collaborationist friends, the theatrical eminences Jean Cocteau and Sacha Guitry, intervened to save him, but the release order arrived too late: He had died of pneumonia the day before.

The protean, multitalented Jacob first arrived in Paris in 1894, where he lackadaisically studied law and art, wrote a bit for newspapers and worked at clerical jobs. In 1901, he met the 18-year-old Picasso, which he regarded as “the greatest event of my life.” The two friends were soon rooming with other artsy types in the dilapidated apartment building Le Bateau Lavoir, sharing meager meals and boldly setting out to conquer artistic Paris. The duo became a trio, however, when they were joined by the ebullient, herculean Guillaume Apollinaire, revered by French majors everywhere for “Le Pont Mirabeau,” that most wistful of modern love poems.

Though a talented painter, Jacob is nowadays best known as a poet. His most famous book, 1917’s “Le Cornet a Des” — “The Dice Cup” — reinvigorated the 19th-century prose poem, a mainly Gallic genre represented by Baudelaire’s angst-ridden “Paris Spleen” and the Comte de Lautreamont’s phantasmagoric “Songs of Maldoror,” the latter notorious for its proto-surrealist imagery: “As beautiful as the chance encounter of a sewing machine and an umbrella on an operating table.” In contrast, Jacob’s dice-cup poems exhibit a subtle mystico-humorousness. Consider “The Feminist Question” in William Kulik’s translation from “The Selected Poems of Max Jacob”:

“Without admitting it, he was afraid she might someday get her animals wrong. When she arrived at the foot of his tower, the frail romantic horsewoman reined in her galloping steed, went inside, gave her fiancé a massage, then whistled for her mount which had wandered off but came back to her. That Mademoiselle de Valombreuse was a masseuse, her fiancé easily forgave, but that she could subdue a beast, that was just too much.”

That other Max — Beerbohm — once composed a mock-Shakespearean skit featuring Lucrezia Borgia, St. Francis of Assisi, Savanarola, Michelangelo and virtually everyone you can think of from the Italian Renaissance. Warren’s biography is sometimes like that. The portrait on the book’s cover was painted by Modigliani. As a child, Jacob was treated for neurasthenia by Jean-Martin Charcot, Freud’s mentor. While a student, he lived in the same apartment building as the Swedish playwright August Strindberg. The Christian philosopher Jacques Maritain became his spiritual adviser.

Over time Jacob came to know almost every important Paris-based modernist: Le Douanier Rousseau, Erik Satie, Georges Braque, Gertrude Stein, Andre Breton, Maurice Sachs, Michel Leiris, Marcel Jouhandeau. He introduced a quickly besotted Cocteau to the teenage Raymond Radiguet, author of “The Devil in the Flesh,” which slightly fictionalized the boy’s scandalous love affair with a married woman. Another teenager, named Albert Camus, sent Jacob a letter from Algeria and received a reply that he said “did me a great deal of good.” And for R&R, where else would a gay Jewish Christian spend a weekend than with the Princess Ghika, formerly known as Liane de Pougy, the most beautiful demimondaine of her generation? Later in his career, Jacob’s staunch friend and editor would be none other than Jean Paulhan, a future leader of the Resistance and the lover for whose erotic delectation Anne Desclos would write, under the pseudonym Pauline Réage, “The Story of O.”

Being a distinguished poet herself, Warren pays particularly close attention to the richness of Jacob’s language and what she neatly calls his “controlled phonetic delirium.” For example, the phrase “Les manèges déménagent” means “the merry-go-rounds are moving out,” but the English loses all the original’s twisty, orthographic wordplay. In “Art Poetique” Jacob proffers one thought-provoking observation after another: “So well written, so well written that nothing is left.” “Personality is only a persistent error.” When a magazine asked, “Why do you write?,” he inarguably answered: “In order to write better.”

Given its length and scholarly detail, “Max Jacob: A Life in Art and Letters” isn’t sprightly, but it is definitive and chockablock with entertaining anecdotes. As a foreigner in Paris, Max’s friend Pablo frequently carried a gun for protection. “Once, in a café, when some chatterbox spoke slightingly of Cézanne, Picasso placed his revolver on the marble tabletop and said, ‘One word more and I’ll shoot.’ ” Legend has long had it that the revolver was a gift to the painter from the playwright Alfred Jarry, author of the absurdist play, “Ubu Roi.” Alas, according to Warren this isn’t true. A pity.

Michael Dirda reviews books each Thursday in Style.

MAX JACOB

A Life in Art and Letters

By Rosanna Warren

Norton. 720 pp. $45