"I am not a biographer, in the usual definition of that term," Ann Powers writes in her introduction to Travelling, describing herself instead as "a critic, a kind of mapmaker". Her book follows Joni Mitchell's trail across eight decades, mapping out not just the artist's singular musical journey, but her misjudgments, musical and otherwise, in a discursive narrative that is peppered with critical theory and personal self-questioning.

"I had to keep uprooting myself, rejecting any settled stances about who this woman is and why her music is so special," Powers says of her often interrogative approach. Her authorial presence, often illuminating, but sometimes distracting, is the defining undercurrent throughout, which means some passages have the feel of someone thinking out loud as they grapple with the more problematic aspects of Mitchell's life and work. Then again, it is her "thorniness", as Powers approvingly calls it, that also makes the singer an even more compelling subject.

The journey begins unpromisingly with a prelude titled A Note on Naming, in which Powers frets over whether to call her subject "Joni" or "Mitchell", the former potentially trivialising, the latter too austerely formal. In the end, she decides to alternate between both depending on the context. This kind of fastidiousness is a constant throughout, but thankfully is put to much better use in Powers's analysis of Mitchell's songwriting and often startling musical inventiveness.

What emerges from the off is an artist whose creative restlessness was matched by a single-mindedness that was sharpened during her late-60s ascendancy as a female singer-songwriter remaking the rules of a mostly male tradition. As part of Los Angeles's fabled Laurel Canyon scene, Mitchell was surrounded by male musicians, including David Crosby, Graham Nash and James Taylor, all three of whom fell for her, but could not compete with her dizzying creative momentum. She could be one of the boys when it suited her, but stood apart from them in terms of her precocious talent (and the way in which she nurtured it). Powers compares her to "the proverbial girl in the playground pickup game who cheers on the boys, but then grabs the ball and throws it in a perfect spiral towards the basket".

Mitchell's greatest songs, as several of her contemporaries attest, often arrived out of a state of rapt attentiveness that was at odds with the communal creative vibe that held sway in Laurel Canyon's post-hippy boho zone. The intense creative introspection that produced Mitchell's 1971 album Blue was a case in point. It was surely honed in the Canadian prairie city of Saskatoon, where she grew up, and where, aged nine, she contracted polio and, for an unnervingly uncertain time, lost the ability to walk.

Powers deftly links her self-willed recovery, which included her "letting herself get lost" in the wide-open spaces near her suburban family home, to the hauntingly symbolic photograph of Mitchell on the inner sleeve of 1976's Hejira, arguably her most starkly beautiful and ruminative album. In Joel Bernstein's black and white image, Mitchell skates confidently across a frozen lake in Wisconsin like a large black bird about to take flight on outstretched wings. "The image doesn't speak of freedom; it speaks of memory," writes Powers. "Being alone in this space has taken the woman somewhere else, where she's been before. In this moment, she could be a child."

Throughout, Powers roams freely and associatively across Mitchell's life and work, often drawing similarly surprising parallels between the artist's past and present. As the title suggests, Mitchell's constant creative journeying was an end in itself, both risky and renewing. It propelled her from her Canadian trad-folk roots to the very centre of LA's singer-songwriter aristocracy and beyond, to more challenging collaborations with maverick jazz musicians such as the precociously gifted Jaco Pastorius, and the elderly but still irascible Charles Mingus, with whom she worked just before his death.

While Mitchell's reverence for black jazz musicians was heartfelt, Powers also delves deep into one of her strangest and most wrong-headed acts of would-be homage. In 1976, she assumed a black alter ego, darkening her face, applying a fake moustache and donning a "brightly banded fedora" atop an afro wig. Basing her look on generic pimp-style characters from blaxploitation movies, and calling herself Art Nouveau, she attended music biz parties without any of her friends and fellow musicians seeing through her disguise. The following year, she released Don Juan's Reckless Daughter, appearing on the cover in the same disguise. As Powers points out, such an act of cultural appropriation seems "inconceivable" today and her forensic interrogation of the unfortunate episode, which draws on essays and critiques by contemporary black scholars, makes for uncomfortable reading.

More intriguing still is Powers's exploration of Mitchell's long separation and eventual reunion with the daughter she gave up for adoption while still a struggling but steely ambitious folkie. "I know the adoption triad intimately," writes Powers, describing how she "raised a child born of a woman even younger than Joni was when the baby she named Kelly Dale was born". It's that kind of book, shifting often unexpectedly from the critically detached to the deeply personal.

In old age, having recovered from a long, debilitating illness and recently, triumphantly, performed on stage again after a lengthy absence, Mitchell has attained an almost saintly status, particularly among a coterie of young, American, female musicians who rightfully revere her as a role model and creative inspiration. Having celebrated her subject's all-too-human thorniness, Powers is not entirely comfortable with this late canonisation, not least because the late "swell of adoration" that attends her tentative comeback has tended to obscure her complexity as a human being and an artist.

There is more than a grain of truth underpinning Powers's refreshingly sceptical refusenik stance, not least because our seemingly insatiable need for this kind of all-pervasive celebratory nostalgia is always somehow diminishing and reductive. In contrast, Powers's book is a counterweight to the myth of "Joni the unimpeachable treasure that many feel so compelled to protect". You may not agree with every critical twist and turn that Powers puts herself - and her idol - through, but the end result may well make you reconsider the Joni you think you know.