Stevie Wonder’s legacy ranks among the most powerful in pop music, though his story remains elusive. His songwriting and his voice echo through virtually all R&B-related sounds that have followed him, from Michael Jackson to R. Kelly to Kanye West, yet there is no major biography, no documentary, nothing that presents the full sweep of the most dominant and defining artist of the 1970s. And make no mistake—it was an era of superstar acts and chart-busting albums, but no one was as universally loved, respected, and honored as he was.

To be sure, Wonder has done himself no favors in getting his story told. Long ago, the media figured out that his world runs on “Stevie Time,” that schedules and deadlines don’t apply to this towering genius, whether that means showing up for an interview or delivering an album.

“You set a goal in your mind,” he told me in 2005, when the record A Time to Love was released after many years’ delay, “and you say, O.K., this is what these songs need to have, this project needs to have, and you don't really settle for anything less than that.” As good as his word, Wonder has not put out a new album since.

But even Stevie Wonder realized that there was one project of his that demanded to be recognized and appreciated while he was still able, and in 2014 he mounted a tour to present, in full, his 1976 magnum opus *Songs in the Key of Life, *which turns 40 next month. The album—two LPs plus an additional four-song EP—was the culmination of a historic period of creativity, a concentrated burst of music matched only by a handful of artists (mid-’60s Bob Dylan, early ’70s Rolling Stones, ’80s Prince). Its ambition and scope were unprecedented, its power and resonance were timeless—and when it was done, he never approached its caliber or impact again.

In 1971, a decade into an already-legendary career, Wonder celebrated his 21st birthday by allowing his contract with Motown Records to expire, holding out for a new deal which gave him a higher royalty rate and creative control over his work. He responded with two albums in 1972, the breakthrough Music of My Mind and his first true masterwork, Talking Book. His next releases, 1973’s Innervisions and Fulfillingness’ First Finale in 1974 (which followed a serious automobile accident that left him in a coma for four days) won back-to-back Album of the Year trophies at the Grammys. Wonder not only reeled off six Top Ten hits in three years and repeatedly pioneered new frontiers in music technology (he introduced the Moog synthesizer and the sampler to the public), he also established himself as a leading voice of social protest with such profound commentaries as “Living for the City” and “You Haven’t Done Nothing.”

And then came the wait. Wonder, expressing frustration with America in the aftermath of Watergate and the never-ending war in Vietnam, had started to talk about quitting the music industry and moving to Ghana to work with handicapped children. He got as far as starting to make plans for a farewell concert. Instead, though, in August of 1975, he signed a new contract with Motown—a seven-year, seven-album, $37 million agreement, the largest deal made with a recording star up to that point. Though there were murmurs about an album release later that year, Wonder first took time off, then kept adding songs to the stockpile he had amassed, then decided there was more mixing and post-production required.

Two years might not seem like a long break between records now, but at the time, it was unheard of, especially from the biggest star in the game. Motown staffers printed up T-shirts that read, “We’re Almost Finished.” And when the twenty-one-song set was finally released on September 28, 1976, the promotional campaign borrowed some pages from the playbook for the previous year’s record-shattering movie Jaws, which had redefined the making and meaning of a true blockbuster.

Motown flew press and everyone who had worked on the album to the Long View Farm studio in Massachusetts for the first playback and a battery of interviews with Wonder, presenting everybody with autographed copies of the album. As it made its way to record stores, Songs in the Key of Life (changed from the working title Let’s See Life the Way It Is) was everywhere you looked. I distinctly remember what a major event the release was, and I was only 10 years old at the time.

Yet this was the very rare case in which there was simply no way to overhype the music. The sound drew effortlessly from funk, pop, jazz, Latin styles. Tracks ranged from the fusion groove of “Contusion” to the delicate voice-and-harp duet “If It’s Magic.”

Though more than 130 musicians are listed in the credits (including such stars as Herbie Hancock, George Benson, and Minnie Riperton), Songs in the Key of Life never strays from the singular, blazing vision of one artist. The sneaky pre-chorus in “Knocks Me Off My Feet,” the transcendent, spiraling rise of “As”—hooks pile on top of hooks, tucked irresistibly and unerringly into breaks, fills, and intros, most obviously on the album’s two biggest hits, the nostalgic, swaggering “I Wish” and the shout-out to Wonder’s jazz forefathers (and mothers) “Sir Duke,” soon to take up permanent residence in the repertoire of every marching band in the country.

The hat tip to Duke Ellington was significant, since in some ways Songs in the Key of Life most recalled that composer’s classic long-form works like “Black, Brown, and Beige” (or Max Roach’s Freedom Now Suite or Charles Mingus’ “Pithecanthropus Erectus”) in its attempt to portray the complete sweep of the Black American experience. The album’s opening lines—“Good morn or evening friends/Here’s your friendly announcer/I have serious news to pass on to everybody,” from the almost impossibly gorgeous “Love’s in Need of Love Today”—laid out the stakes for all that would follow.

“Village Ghetto Land” spoke of the pain and indignity of poverty and homelessness. “Black Man” served as a literal history lesson, calling out examples of how all colors and cultures have contributed to global progress. “Ngiculela—Es Una Historia—I Am Singing" soared triumphantly in Zulu, Spanish, and English, with melody easily uniting all languages. Then there were the simple celebrations of love and faith—on “Isn’t She Lovely,” the magic of birth and family—emotions shared by all, but never to be taken for granted in undervalued Black lives.

The year 1976 was, of course, America’s Bicentennial year, and especially at a time of such frustration and distrust surrounding the government and the country’s institutions, U.S. residents were bombarded with stories of our history and heroics. Songs in the Key of Life functioned as a corrective, a counter-narrative, alongside such other radical, groundbreaking statements as Richard Pryor’s Bicentennial Nigger and Alex Haley’s Roots, both of which were released just a few weeks before the album.

Almost everyone understood the magnitude of Wonder’s achievement, but there were some objections, mostly having to do with the length and sprawl of the record. “[I]t has no focus or coherence,” wrote Vince Aletti in a wildly mixed but mostly favorable review in Rolling Stone. “The eclecticism is rich and welcome, but the overall effect is haphazard, turning what might have been a stunning, exotic feast into a hastily organized potluck supper.” But to complain about the excess was to miss the point—any great double-album (The White Album, Exile on Main Street) could easily be edited into something tighter and more consistent, but the all-encompassing aspiration is the whole idea, the desire to contain multitudes and to cover as much ground as possible during a revved-up creative groove. Sometimes, more is more.

Certainly, the public understood. Songs in the Key of Life entered the album charts at No. 1, only the third record to hit that spot straight out of the gate (after Elton John’s two previous releases). It then stayed there for the rest of the year; to understand just how ubiquitous the music of the mid-’70s could be, consider that it knocked Frampton Comes Alive! out of the No. 1 slot, and was finally bested in January of 1977 by Hotel California. Inevitably, Wonder won his third straight Album of the Year award at the Grammys (he missed the ceremony because he was visiting Nigeria at the time).

After Songs in the Key of Life, though, something seemed to deflate in Stevie Wonder’s work. It was as if, at the ripe old age of 26, he was bored by the idea of just writing hit songs. His next album was 1979’s mystifying, experimental, mostly instrumental Journey Through the Secret Life of Plants, a double-album soundtrack to a documentary about the feelings of greenery and flowers. Turning his attention to the crusade for a national holiday to honor Martin Luther King, Jr., Wonder rebounded with Hotter Than July in 1980 (featuring the sublime “Master Blaster (Jammin’)),” but since then, it’s mostly been long waits in between underwhelming new records.

It was like there was nowhere left to go after Songs in the Key of Life—and maybe there wasn’t. The album was more than just a masterpiece; it was the culmination of all of the potential that Stevie Wonder showed since his days as an 11-year-old prodigy. Musically, politically, culturally, it was the fulfillment of everything that Motown and the ’60s soul revolution had promised. And within a few months, disco was the focus for new Black music, while in the parks and playgrounds of the Bronx and beyond, hip-hop was taking shape for the next generation.

The sound of Songs in the Key of Life never stopped reverberating. Elton John and Prince said that it was their favorite album. Michael Jackson and Whitney Houston sang its praises. Coolio, Ol’ Dirty Bastard, and Will Smith sampled its hooks. Mary J. Blige and Luther Vandross covered its songs. Kanye West said in 2005, around the release of Late Registration, “I'm not trying to compete with what's out there now. I'm really trying to compete with Innervisions and Songs in the Key of Life. It sounds musically blasphemous to say something like that, but why not set that as your bar?” Still, it was a genuine event when Stevie Wonder decided to take Songs in the Key of Life to the stage 38 years after its release, and bring the spotlight back to his greatest musical accomplishment.

It required several dozen musicians on stage to recreate the album’s arrangements, with full horn and string sections (that harp!), but it also imposed a discipline on Wonder’s performance, which too frequently devolve into a mess of medleys and sing-alongs. The shows were magnificent, the words as true as ever, and there was Stevie, still telling us—showing us—that “music is a world within itself/With a language we all understand.” Almost four decades later, you could feel it all over.