What makes Joni Mitchell's music special? The lyrics alone put her on 20th-century music's Mount Rushmore, alongside her cultural mirror Bob Dylan and her brief lover Leonard Cohen. But for me it's her phrasing, her tunings and her sense of time. Decades on, her music remains endlessly surprising. Think a line is going in a certain direction? Think again, as Mitchell bends it away; or shifts key; or arcs her voice into its celestial sphere, only to suddenly plummet, like a plane in turbulence. And yet the swerves feel somehow right, inevitable.

Enlisting the help of jazz greats from Wayne Shorter to Herbie Hancock, Mitchell invented her own musical grammar: one of conversational fluency, perhaps best articulated with the bassist Jaco Pastorious, the pulsing voice behind Mitchell's mid-to-late 1970s masterpieces Hejira and Don Juan's Reckless Daughter.

There was something faintly miraculous about Mitchell from the start. In 1942, Myrtle Maguerite visited a fortune teller in the small town of Alberta, Canada and was told that she would be wed within a month and have a child within a year. Given that most young men were abroad at war, this seemed unlikely. Yet, two weeks later, Myrtle met and married Bill Anderson, an airman, and within a year Roberta Joan Anderson - Joni, as she would name herself - was born.

Never one to be outdone, Mitchell had Lazarus beat: she learnt to walk not twice but three times, overcoming a bout of polio as a nine year-old that left her bedbound for a year, and again after a brain aneurysm in 2015. The polio forced a permanent weakness into her left hand, such that when she picked up a guitar a few years later, she found she couldn't reach the traditional chords. And so she devised alternative tunings and what she calls 'chords of inquiry': unresolved chords whose song hangs like a question mark in the air. From the off, Mitchell sounded different.

Rebirth, resilience, reinvention: all come through strongly in the music critic Ann Powers's Travelling. 'This is not a standard account of the life and work of Joni Mitchell,' the author insists, before giving us a largely chronological, thematic account of the life and work of Joni Mitchell. She also gives us an autobiography of Ann Powers, and a biography of the North American mind in the 2020s.

We follow Mitchell from Canada to Detroit to New York to the 'boys' club' of Laurel Canyon, moving on whenever the net of domesticity closes in. The early years have Mitchell marrying undeniable talent with a less observed knack for using others to her advantage. Her sense of her own exceptionalism runs through the book - 'she was about as modest as Mussolini,' according to the (hardly shy) David Crosby - but she backed it up with a string of innovative records that took her from late 1960s folkie to 'confessional' icon to jazz frontiersman.

We see her ill-fated marriage to Chuck Mitchell; a string of famous lovers-cum-collaborators, from Cohen and Crosby to Graham Nash and James Taylor; her decade-long marriage to the musician Larry Klein and their productive if difficult 1980s; the critical resurrection of her career in the mid-1990s with the Grammy-winning Turbulent Indigo; and her late-life reclusiveness, illness and partial recovery. Along the way Powers explores such sidetracks as the cult of the child in 1960s California, the feminist 'reclamation of sadness', and popular music's legacy machine. The picture that emerges of Mitchell is almost a trope: an artist so committed to her work, she struggles to commit to others. The curiosity that fires the music fires a desire to keep moving.

Powers writes beautifully on the texture of music, particularly jazz, and her book hits many memorable notes, such as on the 'humid, thick, seductive' sound of Hejira, the broader influence of Miles Davis, and the link between Mitchell's painting and her music-making.

However, Powers's grasp of Mitchell's self-fashioning rubs up against her own politics, and can lead to some confusing positions. She is frustrated at the way Mitchell, and women in general, were trapped by 'femininity' in the 1970s, yet also 'a little bit angry at Joni for trying to genius her way beyond gender' and not displaying more female solidarity - despite also dismissing 'womankind' as a category: 'As if that...even really existed.'

True to our time, Powers pays significant attention to racial politics - sometimes illuminating, given Mitchell's 'strong identification with black men', sometimes deeply reductive - and displays her own credentials by self-flagellating over her 'dance moves while drunk'. But she avoids other equally important glosses on the music, even when Mitchell herself has repeatedly discussed them. The most obvious of these is the effect on Mitchell, aged 21, of putting her baby daughter up for adoption - an event which changed the course of her life, influenced her art (by her own admission) and, some argue, even made the songs possible. But on this subject, Powers (herself a mother by adoption) is suddenly coy: 'None of this is for me to really say'; 'every adoption story is different... I would not pry'.

Powers has no qualms about prying into other areas. Indeed, it's hard not to feel that she is trying to protect Mitchell on some questions and out her on others. It makes for a book of uneven rhythm, rich in insight, but one that, like a Joni Mitchell song, conceals even as it confesses.