At the end of the 1990s, Bill Callahan gathered a small group of children to sing along with two songs for his new “album for teenagers,” Knock Knock. The way Callahan told it at the time, he didn’t record in the booth with the selected members of the Chicago Children’s Choir, whose giddy voices accompany his drawled, deadpan musings in “No Dancing” and “Hit The Ground Running.” Instead, the 30-something songwriter stood just outside the glass partition, mouthing along. “They wanted to see me sing,” he explained.

In the two decades since the release of Knock Knock, Bill Callahan has been increasingly acclaimed for his refined exploration of adult concerns. After abandoning the Smog pseudonym to release music under his own name starting in 2007, his records have focused on the thorny questions that accompany getting older. What does true companionship look like? Where do I find peace in this world? How can I be productive while acknowledging the joke that is existence? His answers mostly arrive in his delivery, a voice that’s grown deeper and truer with every record he’s made. Take things slow, he seems to say. Listen closely. Breathe.

By the time he released Knock Knock, Bill Callahan had already been recording music for a decade. His best songs from this period — the earliest of which were crudely recorded, sloppily performed, and rarely consisting of more than just a few words — are characterized by insularity: private moments he took devilish joy in capturing for eternity. In one song, a man listens to AC/DC and does several dozen push-ups in a motel room. In another, he collects his ex’s stray belongings and arranges them just-so on his bedroom floor. Without beginnings, endings, or context to fill us in, these passing images burrow in the subconscious like wrong texts from someone lonelier and more desperate (and way funnier) than us.

On Knock Knock, Callahan began looking outside, and his stories gained momentum. It’s largely an album about escape, however momentary or superficial, reflecting a childhood spent in constant flux due to his parents’ jobs with the National Security Agency. There’s the pensive narrator of “River Guard,” a prison guard who watches his inmates go for a swim and contemplates the metaphor within the context of his own life. In “Teenage Spaceship,” Callahan is a kid walking through his sleepy hometown at night, imagining himself enormous, soaring over all the quiet houses. On a previous album, Callahan might have related more to the folks inside, asleep or oblivious to the imagined invasion happening just outside their windows. Now, he “loomed so large on the horizon,” glowing “beautiful with all my lights.”

His second and final album made with the aid of Jim O’Rourke and his first featuring a full band, Knock Knock was a leap forward for Callahan the musician. He discovered what would soon become the defining principles of his sound — a grainy, homemade strain of folk music that seemed connected to the past without ever sounding reverential. (He claimed to have not heard Leonard Cohen until critics started comparing them.) Arriving between 1997’s breakthrough Red Apple Falls and 2000’s sprawling Dongs Of Sevotion, Knock Knock was the heart of Bill Callahan’s collage phase. He seemed to be making up for lost time with records that bent to his every whim — string arrangements, children’s choirs — without abandoning the rawness that characterized his early work.

While Callahan once compared his musical ambitions to window shopping — something he could only fantasize about from the outside — his songs were now bold and confident and wide open like country roads. The record begins with droning Americana cellos in “Let’s Move To The Country” before Callahan’s own voice enters the mix, mimicking their staccato pulse with his zen baritone. Like a breathing exercise set to music, the song is filled with blank spaces, both in its arrangement and its lyrics (“Let’s start a…./ Let’s have a….”). Always an expert lyricist, he was now interested building sonic narratives by contrasting moments of pure, meditative beauty with gnarlier dissonance. The austere arrangements of tracks like “Sweet Treat” and “Left Only With Love” suggest the loneliness of a dissolved relationship: The former is all pent-up anxiety, while the latter is unburdened like a sigh of relief. Voicing the other perspective is “Held,” a celebration of intimacy so propulsive it would one day soundtrack a car commercial starring Bob Dylan.

At the time of its release, many listeners characterized Knock Knock as a concept album, and it’s easy to see why. “I had to leave the country,” he sings near the end of the album, tying up the travelogue that kicks off in the opening lyrics (“Let’s move to the country/ Just you and me”). If you’re looking for it, you could even separate the album into three distinct acts: an opening suite about finding new love and solace in the natural world; a middle passage about childhood trauma and how adult relationships can trigger those memories; and a long farewell as Callahan skips town, drives forever, and wishes the best for the people in his rearview mirror.

Of course, time has taught us that this is just the way that Callahan writes. So if Knock Knock is a concept album, then so is everything else he’s ever released. “A lot of it is subconscious coincidence,” he told Pitchfork about his writing process. “I think about certain themes in my life and focus on them. That’s what makes a record hold together, because there might be some aspect of a sound or of human nature that I’m thinking about a lot.” Since Knock Knock, he’s released albums with repeated musical motifs (the title tracks of 2001’s Rain On Lens) and recurring lyrics (the various apocalypses on 2011’s Apocalypse). And yet, Knock Knock remains a singular statement — a crumpled map of sad, scenic views, as immediate and inhabitable as anything he’s made.

Released within months of similarly definitive statements from his Drag City/Domino labelmates — Silver Jews’ American Water, in October 1998, and Bonnie ‘Prince’ Billy’s I See A Darkness, later in January 1999 — Knock Knock arrived during a blossoming era for independent songwriters, using their creative freedom to make albums that felt equally scrappy and expansive. Despite the creaks in their voice and poorly tuned guitars (or maybe because of them) their songs felt like anthems. The arrangement of “River Guard” laps slowly like light rain on a small body of water — all mournful piano and slide guitar — but it builds toward something life-affirming. “Every breath,” Callahan sings, “Death-defying.”

Knock Knock is not Bill Callahan’s masterpiece — that would be 2005’s A River Ain’t Too Much To Love. But its songs are among his most enduring, resplendent in their lack of cynicism, even at their bleakest (the visions of domestic abuse in “Cold Blooded Old Times,” the constant trials of “River Guard”). “Some of the themes [on Knock Knock] are things I associate with teenage years — having big plans, thinking you can live like a gypsy,” Callahan told The Chicago Reader. “There’s a lot about moving and traveling on the record. Most adults let that die.” Years later, in a stunning song called “Riding For The Feeling,” he’d sing about the other perspective of all that leaving: the price you pay, the people you abandon, the empty feeling when you’re back alone in your hotel room. It would all come with time. “With every mile, another piece of me peels off and whips down the road,” he noticed in Knock Knock’s dreary penultimate track “I Could Drive Forever.” “I should have left a long time ago.” It’s far from a happy ending, but for now it was enough.