The arrival of the collected letters of Sylvia Plath – this is only volume one; a second will follow next year – provides something of an object lesson in the weird desperation involved in what we might call heritage publishing. Of course we understand that Faber (and in the US, Harper) is thrilled to have this long-dead poetic genius on its list; such pride isn’t misplaced. But whether this means that every word Plath ever wrote, up to and including her scholarship applications, is of interest to anyone other than truffling biographers and PhD students is another matter altogether. Lugging around this rusty anchor of a book – it runs to more than 1,400 pages – what I felt mostly was exasperation. The notion that Plath’s every utterance is sacred would be dumb even if she ranked with Keats or Waugh as one of the truly great letter writers. The fact that she clearly doesn’t – the majority of those in this volume, written to her mother, Aurelia, are marked by their quotidian sameyness – only makes it seem the more vacuous.

It’s a problem that’s hardly helped by their editing. The focus of Peter K Steinberg and Karen V Kukil, the Plath scholars who have devoted long years to this project, appears to be on accumulation, not enlightenment; their footnotes, which they describe as “comprehensive” but in fact tend to the minimalist, are far from adequate. Should Plath mention, say, a story she has read by Jean Stafford in the New Yorker, they will certainly endeavour to identify which one it might have been. But of the connections between the letters and their author’s life and work, they tell us almost nothing. The reader, then, is entirely in Plath’s hands, which is not only tricky in narrative terms – ellipses come as standard in correspondence of which you get to read only one side – but also perilously unbalanced.

Where does the truth, or what passes for it, ultimately lie? The editors offer us no rudder. Plath’s biographers, for instance, have insisted on her difficult relationship with her mother, and certainly no one who has read her poem Medusa – “My mind winds to you/Old barnacled umbilicus” – is likely to doubt that it was at the very least complex. Yet in these letters, she appears not only as a dutiful and loving daughter – on 4 October 1950, she writes to “dearest mummy” no fewer than three times – but as an exceptionally confiding one, too. Here she is, to take one example of hundreds, on Ted Hughes, in a pages-long letter to Aurelia written in April 1956: “… in the sight of all the stars and planets and words and food and people in the world, there are only the two of us who are whole and strong enough to be a match, one for the other… Ted is incredible, mother.” It’s the kind of letter, breathless to the point of hyperventilation, that plenty of daughters might write, but very few would send. At this point, after all, she had known her future husband for a mere eight weeks.

The screeds that Plath wrote to Aurelia, published here in their entirety at last, can be seen as a corrective to Letters Home, the selected volume her mother edited and published in 1975, more than a decade after her daughter’s suicide (and in which Hughes had a hand, having proposed cuts seemingly to protect the living). But though we now get to read several hundred more, it’s hard to say what, if anything, is revealed. Every one is a performance, of sorts. Whether Plath is writing about a heavy cold, a new coat, or her latest date, the tone is bright to an almost monotonous degree; you have to look hard for the cracks, for the shadows that trail us all but troubled Plath more than most. Only on page 654 does her pitch finally change, her voice dropping several decibels when she writes to her friend, Edward Cohen, in December 1953, following her attempted suicide four months earlier (Plath took an overdose in the basement of the family home in Wellesley, Massachusetts, and was not found for three days). Where previously she sounded so tinny, now she is matter-of-fact: “I swallowed quantities and blissfully succumbed to the whirling blackness that I honestly believed was eternal oblivion.”

Even then, it’s not long before she’s back up and running. In February, she tells another correspondent of the “fun” she’s having back at college: “In summary, everybody has treated me just as if nothing had happened, and I feel most at home and casual about the whole episode…” It strikes you that Plath has – in these letters – a positively 21st-century attitude to self care. “I feel I owe it to myself,” she writes, justifying the purchase of a new cashmere sweater. She can be funny; I love the letter she writes when she is working as a nanny in 1951, which is styled as a news report about a teenager who has put one of her charges “down the chromium-plated disposal unit in the kitchen sink”. More often, though, it’s her insecurity that’s on display. First, she’ll bolster herself: “I feel damn lovable and good.” Then, she’ll swerve back: “But I need fifty blessing brutes to tell me so and how drastically I need to be appreciated.” There is something of the auditor in the way she minutely charts her emotional and physical health. But while she’s ever attentive to the inward, only rarely do we see her sharp mind brought to bear on the wider world. What did she make of All About Eve when she saw it on a date in January 1951? We never find out.

Plath’s letters to the men she imagined she loved before Hughes have a certain too-muchness: a risky intensity of feeling that brings to mind a circus knife-thrower marking an outline with her blades. So, too, do the 16 love letters, owned by her daughter Frieda and now published for the first time, that she wrote to Hughes in 1956, the year they married (and the book’s cut-off point). Nevertheless, it is for these last that you should borrow this collection from your library, and to which you should turn once it’s in your hands. They alone make the prospect of volume two seem fully tantalising.

These extraordinary dispatches are born of sexual passion, a wave that Plath – you sense this instantly – has not ridden before. When she is with Hughes, no night is too long, no day without its radiant possibilities. Away from him, she is utterly debilitated, a rag doll on a floor. This is not peace. As we know, it will never be that. But the “law courts” that were once permanently in session in her head, whispering the flaws of every man she met, have at last fallen silent – and it’s impossible, as she toils to explain this, not to be thrilled for her. For a few spellbinding moments, her incandescent sentences snaking on and on, the dreadful apprehension of what lies ahead is all but forgotten.

• The Letters of Sylvia Plath, Volume 1: 1940-1956 is published by Faber (£35). To order a copy for £29.75 go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99