The mid-twenties are a fraught time for the well-educated American male. There beckons the possibility of retreat, toward the warm communal bath that was college; that this can’t be done hasn’t sunk in yet. In the late nineteen-eighties, David Foster Wallace found himself in a confusing situation and he responded confusedly. He had already published “The Broom of the System,” a sensation. His second work of fiction, “Girl with Curious Hair,” he guessed correctly, would not make the same splash. Wallace had attempted suicide and survived. He drank and got high. He was unstable, unhappy, and uncertain.

So he looked backward: he returned to school in the hopes of becoming an academic philosopher, like his dad, and he asked his old Amherst College roommate and best friend, Mark Costello, to rejoin him.

Wallace saw this regression as progress. “I figure,” he wrote to one of his editors, Brad Morrow, of Conjunctions, “if I ever want a mate and kids with straight teeth and command of the language I have to figure out a way to ensure income.” By April, 1989, Wallace, then twenty-seven, had been admitted to Harvard, and he and Costello were ensconced in the section of Somerville called Little Lisbon. As Wallace wrote in a letter to Jonathan Franzen, they were two more “paleface refugees from the prices of H[arvard] Square and Back Bay.”

When you research a book, certain comments people make stay with you. You never know in advance which ones. In the case of my biography of Wallace, “Every Love Story Is a Ghost Story,” a phrase I never forgot was a delicate description that Costello and Wallace’s friend Corey Washington gave me of their relationship at Amherst. It was, he said, “like a marriage.” And so it was, too, in Cambridge. Every morning, Costello, an unhappy and overworked associate at a corporate law firm, left Wallace in his Clearasil-stained bathrobe, legs daintily crossed, pen in hand and a notebook on his lap, to take the T to a real job. “How was your day, honey?” Wallace would call out when Costello returned from his day’s work—irony, envy, love commingled. Towels from Wallace’s multiple daytime showers were spread out around the apartment.

Wallace was a machine of words during this time, and Costello was a writer who was doing a job that wasn’t writing; like Wallace, he had graduated from Amherst with a double-summa, half of which was for a novel. By summer, Wallace was leaving out a partly written essay he was writing on rap music—he may have undertaken it on his own or it may have been solicited by Antaeus—and inviting Costello to add responses. Including Costello was kind, but it was also desperate: grad school was approaching in the fall, and Wallace had no idea how to finish the work. Costello remembers his roommate writing twenty-five hundred words in a day and then tearing them up. They collaborated, section by section, short calls and responses, the article extending into a book.

The result, available again after many years out of print, was “Signifying Rappers.” The book’s original 1990 subtitle, “Rap and Race in the Urban Present,” did not survive the remix, but most works like this don’t come back at all. They get forgotten or rewritten into standard histories, the urgent giving way to the authoritative. Not so with “Signifying Rappers”: its time’s concerns are now preserved in amber. A hybrid work, part reporting (Costello), part rumination (mostly Wallace), part music criticism, and part cultural analysis, it’s studded with signifiers, the emotions around which have by now cooled. Here you experience again the urgent topics of that day: the evil that is the yuppie, the question of whether rap incites violence, the ethics of music sampling. It’s curious how, though in the past, these issues extend into our time. Into the gloaming sadly goes Wallace himself now, as well. Like all of his posthumous publications, “Signifying Rappers” must confront what Costello described to me as “the sense of a conversation interrupted, a phone gone dead,” the ineluctable fact of the now-silent Wallace.

“Chess-by-mail” is how Costello once characterized the collaboration, but if that’s the case it’s clear, rereading “Signifying Rappers,” that the two were playing different games. Costello, now a novelist (“The Big If,” 2002), clearly feels the pleasure of escaping purchase agreements and subordinated debentures to write something he cares about. Lester Bangs was toilet-tank reading in the friends’ apartment, and in Costello’s writing you hear his psychedelic tones, jagged prose to capture jagged tunes, mixed with the prosecutorial briefs Costello was trained to write: “For outsiders, rap’s hard to dissect, easy to move to. The command is: dance, don’t understand; participate, don’t manipulate. Rap is a fortress protected by twin moats of talk and technology.”

This was an adviso Wallace could not have heeded even if he had wanted to. Theoretical, rhetorical, agitated, in these pages Wallace wonders why two white kids should be writing about rap. He wonders whether rap is even music. He wonders whether, if it is music, it betokens the end of all music. But it is when he is doing the mundane work of explaining that the pleasures of Wallace’s prose—those gravity-defying sentences, the fondness for the possessive apostrophe, that knack for the perfect image—come out:

Frequently, the DJ is also the rapper’s foil, offering rap refrains, or sometimes replies to the rapper’s verses in the genre’s use of the venerable convention of “call and response,’ often speaking in rhythmless prose against the rapper’s complexly metered rhyme. The Mozart of this last technique is Public Enemy’s shadow-MC Flavor Flav, who holds his head cocked like Stevie Wonder and wears an alarm clock the size of a dinner plate around his neck....

Bangs, you can see, is in his writing, too, but as a call to joy, a spur to escape the isolated self. In the end, as Costello and Wallace were finishing up “Signifying Rappers,” their antic cohabitation replay came to an end. By the late fall, drugs, drink, and the stunning graduate-student workload sent Wallace to McLean, Harvard’s psychiatric hospital, and from there to a halfway house and a new life with new concerns that would bear fruit in “Infinite Jest.” “Signifying Rappers” was left behind. It is, as Costello puts it in his smart new introduction, a “deeply dorked-out artifact of 1989,” dated but fascinating, and I think the more fascinating for being dated.

D.T. Max is the author of “Every Love Story Is a Ghost Story: A Life of David Foster Wallace,” newly out in paperback.

Illustration by Philip Burke.