At once terminally forbidding and inexhaustibly alluring, Nico’s The Marble Index belongs among those ultra-modernist works that stand aside from their art without regard for the consequences. Like James Joyce’s Finnegans Wake or Picasso’s Les Demoiselles d’Avignon, it seems destined to offer an eternal challenge even to those who choose to fall under its obscure spell. This latest reissue, coupled with Desertshore, its successor/sibling, demonstrates that five and a half decades have failed to dent a vital component of its greatness: an obstinate refusal to explain itself or to succumb to the pattern whereby the avant-garde is absorbed and neutralised by the mainstream. There may never come a time when The Marble Index will not be avant-garde.

DAVID BOWIE IS ON THE COVER OF THE LATEST UNCUT – ORDER YOUR COPY HERE

After Nico’s departure from The Velvet Underground in 1967 and the baroque-folk mish-mash of her debut solo album, Chelsea Girl, it represented a complete self-reinvention, jettisoning not just the reliance on other (male) songwriters but the version of the look of a classic ’60s blonde – the Berlin to Catherine Deneuve’s Paris or Julie Christie’s London – that had brought her work as a model and the attention of lovers from Alain Delon to Brian Jones.

Time spent with Jim Morrison, whom she seems to have met while in Jones’s company at the Monterey Pop Festival, persuaded her to begin writing songs, their lyrics – mostly in her second language – influenced by the Romantic and Symbolist poets. With The Marble Index, its title borrowed from a line by Wordsworth, she emerged as, in the words of Leonard Cohen, one of the few “really original talents in the whole racket”.

Nico’s admirers have always heard the music they want to hear. For the majority of them, that’s the anti-glamour of a massively indifferent Gothic existentialism with its roots in the bombed-out despair of wartime Berlin, nurtured in a darkly glittering artistic milieu and fueled by dangerous drugs. Jac Holzman, who signed her to his label in 1968, remembered Frazier Mohawk (formerly Barry Friedman), the man he commissioned to produce her Elektra album, saying that her songs weren’t something you listened to but a hole you fell into.

Holzman hadn’t liked Chelsea Girl but he liked what he heard now: “Nico had a fine contralto voice and a vibrato that pulsed softly but rapidly. Most vibrato bothered me. Hers didn’t.” Curiously, he heard in her music an echo of Jean Ritchie, the dulcimer-playing folk singer from Kentucky’s Cumberland mountains whose own debut album had been the label’s second release, back in 1952.

Holzman and Mohawk hired John Cale, her former Velvets colleague and another ex-lover, to arrange and play on The Marble Index, giving him only four days in a Los Angeles studio but free rein to surround Nico’s songs and the drone of her portable harmonium with all the instruments and effects that took his fancy. From the melting music box of the 59-second instrumental opener, “Prelude”, and the silvery dissonances of the following “Lawns Of Dawn”, the sound-pictures proceed to “Frozen Warnings”, where Cale’s layered violas sound as though their strings are being vibrated by a chill wind from the steppes, and “Evening Of Light”, in which his clanging bells and jagged bowed bass accentuate the imperturbability of Nico’s delivery.

There are two extra tracks on a limited seven-inch, both first heard on a 1991 reissue. The first is the gorgeous “Roses In The Snow”, a folk song from a different world, ebbing and flowing in some strange half-light. The second is “Nibelungen”, its title referencing a German epic poem from the 12th century. Here Nico’s voice is heard by itself, providing evidence in her tone, phrasing and vibrato of the expressive control she had acquired. Given that the original release of The Marble Index contained a mere 31 minutes of music, it’s mildly astonishing that room wasn’t found for these two exquisite tracks. But then Nico generally knew what she wanted, and the original eight-song album certainly conveys a sense of distilled perfection. Mohawk later claimed the credit for that, suggesting that he and the engineer had edited it down in the final mix to what he thought was enough for any listener to take on.

Its startlingly plain black and white cover, using a bleached-out Guy Webster portrait photograph in which the singer’s helmet of dark hair and emphatic cheekbones frame an expression of enigmatic challenge, could hardly have demonstrated a clearer break from her previous publicity shots. In itself it suggests a lack of compromise and the absence of a desire to ingratiate.

While happy to have it on his label alongside such other cutting-edge artists as Love, The Doors and Tim Buckley, Holzman showed no interest in a follow-up to an album that had sold only a handful of copies. For the next couple of years Nico drifted between New York, London and Paris, beginning intense relationships with the film director Philippe Garrel and with heroin, the latter influencing much of the rest of her life. But in 1970 the producer Joe Boyd, now working for Warner Brothers in Los Angeles after starting the careers of Fairport Convention, Nick Drake and others in the UK, persuaded his boss, Mo Ostin, to let him put Nico and Cale back together for a reprise.

The result was Desertshore, recorded in London: a more lyrical, less shocking album, but one full of Cale’s imagination as well as songs inspired by Brian Jones (“Janitor Of Lunacy”), Andy Warhol (“The Falconer”), her recently deceased mother (“Mütterlein”) and Ari Delon, her eight-year-old son (“My Only Child”). Ari himself is heard singing “Le Petit Chevalier”, from one of Garrel’s films. In a sign of Nico’s increasing skill and ambition, some of the songs even have second sections.

This time, Cale’s classical training is more in evidence. The medieval trumpets that decorate “Mütterlein” and “All That Is My Own” are the most startlingly vivid instrumental touch, contrasted by an angelic choir and a piano that rumbles like a bombing raid on the former and by what sounds like music for a Felliniesque carnival on the latter, which also features Nico’s soft-spoken recitative, a contrast with her powerful singing. “My Only Child” is performed mostly a cappella by Nico with a small choir. There is a stark pathos in the piano ballad “Afraid”, which contains the line – “you are beautiful and you are alone” – that provided the title for a recent biography by Jennifer Otter Bickerdike.

Like its immediate predecessor, Desertshore enjoyed negligible commercial success – certainly not enough to persuade Warner Brothers to offer her another album. Neither did it achieve the same critical status, although some listeners found it easier to appreciate, touched by the feelings of loss and regret that come through much more clearly than the emotions so opaquely expressed in The Marble Index.

There would be a final part of what turned out to be the Nico/Cale trilogy: The End, recorded for Island in 1974 and containing the version of The Doors’ title track with which she paid homage to Morrison, under whose influence, during their affair in 1967, she had begun to write the songs that refashioned her image, deepened her mystery and secured her legend. With The Marble Index and Desertshore, it forms a sequence unlike anything else in popular music, unclassifiable and inimitable but the inspiration for much that followed.