Lisbon is a wonderful city for poets and stray dogs. I was a little of both when I lived there for a year in my 20s. It was 2009. I was there on a Fulbright translating Portuguese poetry and wandering its ancient streets.

Every morning, I walked the three cobbled blocks to my favorite café for coffee and a sweet roll, bought a handful of oranges on my way home from a man whose store was never more than half-stocked, climbed the flight of stairs to my apartment, squeezed the oranges for juice, and sat down to the maddening task of translating poetry for a few hours. That done, I burned off my frustration by walking the city.

My walks sometimes took me past a bronze statue of the modernist writer Fernando Pessoa sitting stiffly at a table outside the landmark A Brasileira café with its opulent art deco design. Often tourists posed for photographs sitting on his lap or in the chair next to him as if in conversation, though I don’t think Pessoa would have liked that. He had a horror of being photographed and hated mirrors. “Man shouldn’t be able to see his own face—there’s nothing more sinister.”

Pessoa wrote that sentence, or rather Bernardo Soares, a “semi-heteronym” as Pessoa called him, wrote it in Livro do Desassossego, which English-language readers know as The Book of Disquiet. New Directions recently published a beautiful new edition, translated by Margaret Jull Costa and with an attractive cover by Peter Mendelsund.

The book consists of 438 short chapters—better to call them entries, perhaps, and evoke the diary that this book so clearly resembles. “I look like a rather dull Jesuit,” Soares says in entry 230, as he confesses his mixed feelings about a group photograph taken in his office. On the next page: “I feel like some living creature being carried in a basket on someone’s arm, between two suburban stations.” The entries range in length from one sentence to several pages. (The 2002 Penguin edition, edited and translated by Richard Zenith, the one most English-language readers know, has 481 chapters.) There is no agreement on what a complete edition of the book would consist of or how it would be ordered. Like Pascal’s Pensées, order is entirely conjectural and so up to the editor.

The Book of Disquiet is a seminal work, not only in the history of Portuguese literature but also in the larger universe of literary modernism, but its first edition did not appear in print until 1982, 47 years after Pessoa died. At least two large trunks were discovered after his death; they were filled with thousands of pages of poetry and prose fragments, among them hundreds of scraps written in notebooks, on envelopes and advertisements, and in the margins of texts and labeled “L. do D.” It took decades to assemble the book’s fragments into something passably whole.

There is also the deeper mystery, and editorial challenge, of the numerous alter egos, or heteronyms, from which Pessoa wrote. These were characters with biographies and writing styles so distinct from one another—in diction, grammar, and tone—that they transcend mere personae and take on separate existences. Most of Pessoa’s oeuvre consists of poems, essays, and letters written by these heteronyms. Stranger still, some correspond with one another; they write introductions and elegies for one another; the English ones translate the Portuguese ones. There are poets, philosophers, astrologists, a baron, a French satirist, a hunchbacked girl who writes a letter to a metalworker.

Why Pessoa chose to write with heteronyms is not clear. No consensus has ever emerged. The impulse among scholars is to look elsewhere for the necessary diagnostic tools—to psychology, biography, history. Perhaps it was something to do with his brain chemistry, or some childhood trauma was at work, or maybe the cultural trauma of modernism where, said Marx, everything that is solid melts into air. Maybe the whole thing was a joke. Literary history is no guide, because there was no precedent for the heteronyms in Portuguese literature or anywhere else. What if instead of looking elsewhere for an answer we looked inside ourselves?

Pessoa’s major heteronyms are mostly poets. They include Alberto Caeiro, a keeper of sheep, who wrote pastoral poetry in a deceptively plain manner, and his two disciples, a lapsed classicist named Ricardo Reis and the Whitmanian Álvaro de Campos. Then there is Bernardo Soares, the semi-heteronym, closer to Pessoa himself than the others. Soares too was a poet, but mostly he wrote prose, which he collected and endlessly reorganized into The Book of Disquiet.

“Living seems to me a metaphysical mistake on the part of matter,” Pessoa writes. And: “No ideal merits the sacrifice of one toy train.” The book is composed of such maxims, as well as recriminations, private thoughts, weather reports (“this exquisitely blue sky fading into pale pink on a soft, steady breeze fills my consciousness of myself with an urge to scream”) and even the occasional scene. Soares calls it “a plotless novel,” an “autobiography without events,” “a form of prayer,” a “book of random impressions,” “my humble attempt simply to say who I am.”

The Book of Disquiet felt foreign to me when I first read it two years before moving to Lisbon, a gift from an Iowa Writers’ Workshop classmate who had studied poetry at Pitt with Tomaž Šalamun and seemed part of a more cosmopolitan tradition than the rest of us, busy choosing between Pound and Stevens. The Book of Disquiet’s digressions, the author’s lack of interest in situation, and the ease with which he traffics in the immaterial and the abstract were all strange to me, as was the melancholy and the aura of irretrievable loss—the Portuguese call it saudade—that permeate the book. Now, whether from living in Lisbon or through the mysterious alchemy of translation or maybe just because I’m older, in middle age, that aura is a little more familiar. Rereading the book, I encounter an otherness in its pages that has become, somehow, my own.

Desassossego. Pessoa-as-Soares must have chosen the word as much for its music—that sibilant, hissing middle—as its paraphrasable meaning. Disquiet, he says, is “an impatience of the soul with itself.” The word has more emotional range than simple melancholy and more religiosity than our reflexive, all-purpose anxiety. Deep dissatisfaction is both the subject of the book and the shape it takes, since The Book of Disquiet has not found and never will find its final form. It’s that old rag, desire, with its impossible demands. There is a wish to become something else. (Is that always what want most wants?)

Not a wish: a directive, an inevitability. “Everything in me is a tendency to be about to become something else,” says Soares and later, on that same page, “I’m two people who mutually keep their distance.” This tendency expresses itself not just in the infinitely unsettled shape of the whole but also in the tenor of the sentences. This is prose that aspires to be poetry, that wants to be song.

Often, Soares uses the language of poetry to describe his book. He is “weaving music about [his] complaints,” he writes at one point and, later, “sing[ing] vague songs” while he waits to die. “The needs of an internal rhyme, a shift in rhythm,” he admits, sometimes determine the shape and content of his sentences. At one point, he describes the collar of his work shirt as “turned unselfconsciously up around the neck of a poet.” And when he puts himself in the company of other writers, they are always poets: António Nobre, whose book Só, Soares says, will no longer be the saddest book in Portugal; Cesário Verde, also a bookkeeper; Alberto Caeiro, who, unlike Nobre and Verde, is fictional but still managed to produce a good deal of poetry.

Sometimes, as in Japanese haibun or Ecclesiastes or some of Hamlet’s speeches, prose sits so close to poetry—within earshot—that it takes on certain of its qualities. At other times, poets write unlineated verses that privilege image and sound over narrative and argument and call them prose poems. Here, though, is something different, a kind of lyrical prose written by a lonely assistant bookkeeper in a fabric warehouse who, at night, “dons [his] courtly robes in order to write in secret.” His writing consists of poetic evocations of Lisbon, approaching storms, dawn and dusk, and also, and more so, evocations of thought itself, the “external substance of my consciousness of myself.”

Lyrical, poetic—but to call these paragraphs prose poems would be misleading. There is something necessarily prosaic about them. The Book of Disquiet is caught up in the steady drumbeat of ordinary life and all its detritus: a favorite pair of boots, a type of pant that’s in fashion, the way people in Lisbon pronounce Trás-os-Montes, but most of all the ordinary noise of the self thinking about itself. Its pages, like Pessoa’s trunk, are thick with thoughts. The book reads like a diary.

Yet something within it pushes against that easy classification. It edges toward metaphor and silence, those two essential features of poetry. Yes, Soares and his book are two, and those two keep their distance. Poetry? Prose? The Book of Disquiet is both and neither. It is Bernardo Soares returning to the office from his lunch break and being seized by “an overwhelming sense of tenderness” for the objects on his desk and for his colleague’s stooped shoulders. “I might just as well lavish it on the smallness of my inkwell as on the grand indifference of the stars,” he says.