This unjustly overlooked concert album captures her creative zenith

IN 1979 JONI Mitchell was at the pinnacle of her extraordinary powers - but, looking at her career trajectory, you wouldn't have known it. After what looked like a major commercial breakthrough in 1974, with the album "Court And Spark", her sales dwindled with each successive LP. She was never what is known as a "singles act", but where she had once nonetheless achieved hit songs, by 1979 she seldom so much as scraped into the lowest reaches of the charts.

She had gained an audience through folk music, although her form of folk music seemed to occupy a different and more rarified imaginative realm to that of even her most gifted contemporaries. Her most cherished work remains "Blue" (1971), a folk-pop album that became a totem for succeeding generations of young women and defined the confessional songwriting mode. As she turned from folk to jazz in the mid-1970s, she may have gained a new audience, but it was noticeably smaller and less ardent than the one she was losing.

If Ms Mitchell cared one whit about any of this, she never gave the slightest sign of it. It is hard to think of a major artist who so single-mindedly pursued their vision regardless of the cost. "Court And Spark" had placed Ms Mitchell on the threshold of superstardom and it would surely have been well within Ms Mitchell's capability to consolidate the record's success with a similar and equally strong follow-up. Yet Ms Mitchell has never chosen the comfortable option; this is one of the keys to her genius.

Instead she released the first in a pair of fan-confounding masterpieces. First came "The Hissing Of Summer Lawns" (1975), an album as jazzy as it was folky and as avant-garde as it was melodic. Its songs were no longer diary entries by other means; rather they were near-perfect pieces of short narrative fiction (chiefly concerned with the trials of womanhood), reportage or philosophical rumination. Next came "Hejira" (1976), thick with mystery and atmosphere, the account of a personal journey, both actual and spiritual, across the North American continent.

She followed "Hejira" with radical experimentation on "Don Juan's Reckless Daughter" (1977), and the out-and-out jazz of "Mingus" (1979), a collaboration with a giant of the double bass, Charles Mingus. It was on the subsequent tour for "Mingus", at the Santa Barbara Bowl, that Ms Mitchell recorded perhaps the most under-regarded album of her pomp: a live set, "Shadows And Light", released a year later, in September 1980.

"Shadows and Light" barely acknowledges the music Ms Mitchell made before 1975. The original track listing features only two earlier self-penned songs: the fan favourites "Woodstock", with which she closed the concert, and "Free Man In Paris". "Woodstock" is one of those songs - see also John Lennon's "Imagine" or Bob Dylan's "Blowin' In The Wind" - that stand as sentimental, trite and hugely popular anomalies in their creator's catalogue. In her reworking of it, Ms Mitchell offered a clear message: even the most beloved relic of her hippie past was not sacred. This new mode was who she was now.

"Shadows And Light" is the apotheosis of that mode. Ms Mitchell never had a better backing band than the one she took on the "Mingus" tour, including Jaco Pastorius on bass, Pat Metheny on guitar, Michael Brecker on the saxophone, Lyle Mays on the keyboard and Don Alias on percussion. This was the cream of the jazz-fusion crop, a band that combined technical mastery with sensitivity and invention. It could play anything with seeming nonchalant ease: the most intricate numbers from "Mingus", the languid magic of "Hejira", or the souped-up rock'n'roll revivalism of "Raised On Robbery" - which was excluded from the original double LP but appeared on the accompanying concert film, directed by Ms Mitchell.

Instead of closing with "Woodstock", the "Shadows and Light" film ends on its title track, a song quite unlike anything else Ms Mitchell has ever done. It is aural chiaroscuro, a beautiful metaphysical meditation on nuance and complexity. The original version was an avant-pop solo piece, created with studio effects. In concert, Ms Mitchell performed "Shadows And Light" with a five-piece a capella group called the Persuasions, giving it a new, organic richness. It was the final act of a decade in which she was rivalled for consistent brilliance and audacious reinvention only by David Bowie, and which she ended on a high that deserves to be better remembered.