FOR MOST OF the 1970s, Joni Mitchell did not bother giving interviews. Journalists had spent years parsing her songs for intimate personal content, reading every line as a confession; why indulge them with the inconvenience of truth? When The Hissing Of Summer Lawns was released in November 1975, she wrote in the sleeve notes, "The performances were guided by the given compositional structures and the audibly inspired beauty of every player. The whole unfolded like a mystery. It is not my intention to unravel that mystery for anyone, but rather to offer some additional clues." What followed were a few brisk acknowledgements rather than solvable riddles, and a weird creation myth for one song, Don't Interrupt The Sorrow, said to have grown into a "child" who is "rebellious and mystical and insists that its conception was immaculate."

Rebellious. Mystical. Absorbed by the possibilities of change and improvisation. Cryptic where once she was explicit to a fault. This is how Joni Mitchell made her escape from expectations after the commercial zenith of Court And Spark. First, she would unburden herself from the weight of autobiography, be liberated from the stereotypes attached to a female folk singer. Jazz would ensue, and masquerade. At some point, she would leave Neil Young's beach house, jump in a car and disappear into the American wilderness, the Refuge Of The Roads. A new word would enter her vocabulary: Hejira, the Arabic noun to describe the prophet Muhammad's flight from Mecca to Medina. "It said exactly what I wanted," she told Rolling Stone's Cameron Crowe in 1979, the vow of silence finally lifted. "Running away, honourably."

Crowe's interview unravelled through a sequence of glamorous Los Angeles locations: in the office of her manager, Elliot Roberts; at the studio of photographer Norman Seeff; under a hairdryer at a Sunset Boulevard stylist; in her Mercedes, and by her pool. But The Hissing Of Summer Lawns captured a certain scepticism about LA, and about the lives of quiet desperation being played out in the suburban sprawl of southern California and beyond. "I thought, I'm not going to be your sin eater any longer," said Mitchell. "So I began to write social description as opposed to personal confession."

Her characters enacted significant dramas. Don't Interrupt The Sorrow portrayed a housewife fighting back against male hegemony; as in the kitchen, so in art. The music, too, dragged the LA Express musicians a few steps beyond what they had achieved on Court And Spark, finding new and supple ways to track the unorthodox trajectories of Mitchell's songwriting. On Don't Interrupt The Sorrow, the elaborate melody seemed to work itself out in real time, equivalent to how her finely-wrought poetry presented itself as stream of consciousness.

BY THE TIME the Hissing Of Summer Lawns tour was over, Mitchell had something else to put behind her; the relationship with her drummer, John Guerin, which had inspired the most romantic exclamations of Court And Spark. Setting off from her temporary base at Neil Young's house, she drove across country with friends, then made her way back to LA alone by the most circuitous of routes, writing songs on her guitar as she went. "I'm porous with travel fever," she sang on Hejira's title track. "But you know I'm so glad to be on my own." On the album sleeve, a two-lane blacktop was superimposed over her guts.

Hejira was a return, of sorts, to first-person songwriting, but Mitchell's authorial voice, and the dilemmas it interrogated, were now a continent's-width away from singer-songwriter convention. Motion was a catalyst; the songs turned out be internal travelogues more than external ones, conflicted meditations on relationships and responsibility. "Well there's a wide wide world of noble causes/And lovely landscapes to discover," she mused on Song For Sharon, a tour de force of doubt and resilience, written - astonishingly - under the influence of cocaine, "But all I really want to do right now/Is find another lover." Was freedom, so hard-won, really worth it after all?

The answer, Hejira implied, was an equivocal yes. The relative commercial failure of Summer Lawns had given her more latitude to experiment, but the nine songs on its follow-up carried their radical intentions lightly. The music was pared-back, and her small band - still including Guerin, with bassist Jaco Pastorius a conspicuous new addition - manoeuvred in the environs of jazz, but with the focus on serving the dynamic requirements of her poetry.

It was an uncannily empathetic sound, and one which decisively removed her from pop discourse. "I'll tell you, in the last three years I have been very narrow," she admitted to Cameron Crowe in 1979. "I was concentrating mostly on jazz, modern classical music, Stravinsky, polyphonic music. During that time I developed a lack of appreciation for pop music." On 1977's double, Don Juan's Reckless Daughter, that disdain manifested itself in both avant-garde composition and ecstatic release. Paprika Plains mapped out a new extended topography where, over 16 compelling minutes, a Mitchell piano ballad could be tested by Pastorius's questing bass line, soprano sax interventions from Wayne Shorter, and a severe orchestral score courtesy of Michael Gibbs, a jazz composer who had recently been musical director for The Goodies. Dreamland, meanwhile, acted as a poly-rhythmic sequel to Summer Lawns' The Jungle Line, a joyful investigation of cultural colonialism featuring Chaka Khan: "It's a long long way from Canada/A long way from snow chains."

But that globalist perspective, and a desire to shine a light on non-white art, was complicated by Mitchell's unmediated, and sometimes controversial, impulses. For unclear reasons - to satirise the stereotyping of African-American men would be the most charitable reading - she appeared on the front cover of Don Juan's Reckless Daughter in blackface, disguised as a pimp character she would christen Art Nouveau. One unintended consequence was that the cover - and the out of tune strings on Paprika Plains - caught the attention of the wheelchair-bound Charles Mingus, a jazz éminence grise battling the later stages of Lou Gehrig's disease.

Mingus proposed a collaborative setting of TS Eliot's Four Quartets, then, when Mitchell quailed, gave her six melodies to develop into an album. The transition was complete: from working with pop and jazz-adjacent sessioners LA Express; via Pastorius and his fusion comrades in Weather Report; to the court of a true jazz pioneer. However, as Mingus's health deteriorated (he would die just before the album was completed), and Mitchell made constant adjustments to her line-up at the sessions, the implications of the job became overwhelming. How could she do justice to the last creations of a giant?

Mitchell's escape route through the late '70s had relieved her of pop imperatives, freed her to make some of her very best music, and earned her the respect of artists she genuinely admired. But ironically, that last validation resulted in an album, Mingus, whose beauty was undermined by, of all things, a sense of compromise: the presence of one genius, Mingus, had diluted the potency of another. She would tour Mingus in the autumn of 1979, and allow one last elevated jazz unit to reconfigure her songs on the subsequent live album, Shadows And Light. Then the 1980s would begin, and it would be time for Joni Mitchell to adopt a very different perspective.