BLAME BOB DYLAN. The man with buried treasure around every corner. His hugely successful Bootleg Series spawned high demand for the vault openings of the worthies. Neil Young eventually saw the benefit of showing us his workings. Now Joni Mitchell's doing it.

Most Joni fans I know are content to wallow in her marvellous, officially sanctioned catalogue because, going by the material in this bigger-than-it-need-be collection, Joni, unlike Dylan, has mostly been an astute judge of her best work. She has a painter's instinct for knowing what mark to make and when to leave the canvas alone, she has clarity of tone and conjures pin- sharp imagery. If Dylan is all about sprawl and obfuscation, Joni is surely about concision and aptness.

So faced with five CDs of imperfection and repetition, it's tempting to ask: What's the point of all this?

Well, obviously, it's fascinating to hear things that have entered into the realm of legend, like her pitch for the title song to the movie, Midnight Cowboy. It's here in two demos, one a bit awkward, one more assured. It's not bad, but it's hard to imagine it having the impact Everybody's Talkin' had, so someone was right not to pursue it. There's a beautiful unreleased song, Come To The Sunshine (not the Van Dyke Parks tune), and the legendary Hunter, left off Blue. With its multiple strummed acoustics and subtle drumming, Carey-style, it feels instantly familiar, but also too close to a couple of Blue's other tracks. She also dropped the fine Urge For Going, her "protest song at the coming of winter". Each of the 10 she chose brought something unique to the table. Arguably, these wouldn't have. Shelving them helped make Blue perfect.

As did abandoning an alternative approach to River, a copper-bottomed masterpiece, but here with oddly tentative French horns burbling at the end. They make it more Christmassy, perhaps, but otherwise add little value. Removing them was a smart decision. You can hear Joni implicitly making such assessments throughout this collection. A song called Jeremy - an incomplete early attempt at which appeared on Vol. 1 - is demoed, played live, and recorded for Song To A Seagull, but never released. It's a bleak ditty about someone jailed for drug use, and maybe she realised that her take on crime and punishment was a little romanticised, a little bit patronising.

She records Conversation for both Song To A Seagull and Clouds and rejects it both times. This, with previously discarded songs Blue Boy and The Priest, finally makes it onto Ladies Of The Canyon, but it's one of her duller songs. Part of her knew that.

There are also scrappy tapes recorded in her friend Jane Lurie's apartment in New York's Chelsea district (as in "Woke up, it was a Chelsea Morning"), including performances where her voice is inaudible, and bits of conversation that you won't listen to much. Some off- air fan recordings of a John Peel session for the BBC, with stalwart British arranger John Cameron (Donovan, Bobbie Gentry) providing accompaniment, are disappointingly lo-fi, though they offer an interesting anomaly: early Joni with a backing group.

Indeed, the enduring image of Mitchell - flaxen- haired, flutey-voiced hippy goddess alone with her guitar or piano - only lasted for the few years covered by this box. Once she'd switched to Asylum Records, and after the transitional For The Roses (a more oblique Blue), she landed on her future direction with the jazz-burnished sophistication of her second master- piece, Court And Spark, a truer direction for a woman nearing 30, raised on Lambert, Hendricks & Ross (not a brand of cigarettes), who loved to dance. The new style required skilled, supple musicians around her. She'd never again go back to being alone in the studio.