The best friend of the unnamed narrator of Anna Burns’s third novel, “Milkman” (Graywolf), the winner of the 2018 Man Booker Prize, sits her down in a night club to address some behavior. The narrator has alienated their neighbors in an equally unnamed, obviously Northern Irish city sometime in the late nineteen-seventies, with the Troubles in full fury. “It’s disturbing,” the friend explains. “It’s deviant. It’s optical illusional. Not public-spirited. Not self-preservation. Calls attention to itself and why—with enemies at the door, with the community under siege, with all of us having to pull together—would anyone want to call attention to themselves here?” She must stop it, and she must stop it now. The deplorable conduct in question consists of reading books while walking down the street.

“Milkman”—told in an unspooling, digressive, and fretfully ruminative manner that bears a rough semblance to stream of consciousness but is much easier to follow—is set in an urban war zone where carrying around plastic explosives seems less aberrant than using the sidewalk as a study. Yet the conflict that most preoccupies this novel flares not between republicans and loyalists or between Catholics and Protestants—Burns, who grew up in North Belfast, uses vague aliases like “renouncers” and “the opposite religion” to take the edge off the novel’s historical specificity—but between the girl and her community. Like so many such insular, embattled enclaves, her “area,” as she often refers to it, is suffocating and inescapable. The characters go nameless, identified only by their relationships to one another. There’s “first brother-in-law,” a rumor-spreading creep; “third sister,” who seems to spend most of her time getting sloshed with her girlfriends; “maybe-boyfriend,” with whom the narrator enjoys a wary intimacy; “ma,” who is Ma and therefore won’t stop nagging her about getting married and producing babies; and the narrator herself, called “middle sister.”

Middle sister’s best friend has a point. So complete is our narrator’s rejection of her surroundings that she sticks to eighteenth- and nineteenth-century literature: “I did not like twentieth-century books because I did not like the twentieth century.” You can’t really blame her, though. The paramilitary renouncers who effectively run her district have degenerated into quasi-gangs, turning up in Halloween masks and balaclavas to appropriate residents’ belongings “for the good of the cause and the defence of the area.” The police, serving the interests of the “country ‘over the water,’ ” are even worse; at one point, they slit the throats of the neighborhood dogs. “The only time you’d call the police in my area,” middle sister explains, “would be if you were going to shoot them, and naturally they would know this and so wouldn’t come.” The sick or the injured think twice about going to the hospital, because any involvement with the authorities could lead to being forced to turn informant—or just appearing to have been turned—and “either way sooner or later, courtesy of the renouncers, your corpse would be the latest to be found up an entry with a tenner in its hand and the bullets in its head.” But what bothers middle sister most, what she finds hardest to elude, is the “intense nosiness about everybody,” which, of course, preëxisted all of that. Everyone’s behavior is monitored for lapses in respectability, not just the politically dubious but also “certain girls” who cannot be “tolerated if it was deemed they did not defer to males, did not acknowledge the superiority of males, might even go so far as almost to contradict males, basically, the female wayward, a species insolent and far too sure of herself.”

Seething with black humor and adolescent anger at the adult world and its brutal absurdities, “Milkman” wedges itself too deeply in middle sister’s psyche to resemble a wandering city novel like “Ulysses.” Instead, the way that Burns’s clauses trace the switchbacking self-consciousness of social life in her community recalls the mental torments that often seize David Foster Wallace’s characters: “Just as most people here chose not to say what they meant in order to protect themselves, they could also, at certain moments when they knew their mind was being read, learn to present their topmost mental level to those who were reading it whilst in the undergrowth of their consciousness, inform themselves privately of what their true thinking was about.” A novelist can get lost in such labyrinths, but the saving grace of “Milkman” is a tensile story line involving the title character, a forty-one-year-old married local who’s reputed to be a major player in the paramilitary groups. He starts turning up when middle sister is walking (and reading) or running in the park. He offers her rides, displays detailed knowledge of her routine, and eventually begins to make conversation by suggesting that maybe-boyfriend might have an unfortunate run-in with a car bomb in the near future.

The source of this man’s moniker baffles middle sister: “He wasn’t our milkman. I don’t think he was anybody’s. He didn’t take milk orders. There was no milk about him.” Soon she realizes that he intends to take complete possession of her. The sinister delicacy of this campaign—the milkman never touches her and rarely looks directly at her during their encounters—weaves through the novel, keeping it from meandering too far off into its narrative byways. Middle sister’s perspective makes the milkman’s “encroachment” seem terrifying, implacable, and yet frustratingly foggy. In her “hair-trigger society,” she explains, “if no physically violent touch was being laid upon you, and no outright verbal insults were being levelled at you, and no taunting looks in the vicinity either, then nothing was happening, so how could you be under attack from something that wasn’t there?” As far as the neighborhood scandalmongers are concerned, however, she’s been spotted talking to him in the street once or twice and must therefore already be his mistress. Everyone in the novel seems intent on wearing middle sister down, jamming her into a mold that makes more sense to the community—I.R.A. moll, adulterous hussy—than does a girl who reads while she walks. Her response is to clam up, presenting “a terminal face—nothing in it, nothing behind it, a well-turned-out nothing” that “I thought would bemuse the gossips, confound them.”