Volume one of the collected letters of Sylvia Plath – one of the most original poets of the 20th century, and a prolific correspondent – ended with her marriage, while studying at Cambridge on a Fulbright scholarship from America, to fellow poet Ted Hughes in June 1956. The second volume begins with her 24th birthday in October. The new Mr and Mrs Hughes are penniless and without a home of their own, but she has absolute faith in him as a writer and human being. He is “a genius”, the best poet “since Yeats & Dylan Thomas”. Inconveniently, he is also unpublished, and has no strategy for getting into print – but Plath is equal to the challenge. She is an old hand at approaching poetry magazines in Britain and her native US and promptly sets herself up as his agent.

By the start of 1957 she has typed up and submitted Hughes’s first book of poetry to a major poetry contest, which he wins. By 1961 her first collection is forthcoming from Heinemann, his second is out with Faber, and they have a daughter, Frieda, with another baby on the way. They have bought an ancient thatched house in Devon – Hughes has always wanted a home in the countryside – and are fixing it up, intending to live off their own land in a bucolic writers’ Eden. “Ted & I had nothing when we got married, & no prospects,” Plath exults to her mother in America before the birth of her son Nicholas in January 1962, but “in 5 years all our most far-fetched dreams have come true”.

The new Mr and Mrs Hughes are penniless and without a home of their own, but she has absolute faith in him as a writer

And then, five months later, while Plath is still in raptures over their “thatch, acres of apple trees, daffodils, laburnum, owls, bees”, the genius throws it all over. That summer he begins an affair with Assia Wevill, the wife of a friend. When Plath gets wind of it he tells her that he does not want a marriage and children at all. He says, she relates incredulously, that “he was just waiting for a chance to get out, that he was bored & stifled by me, a hag in a world of beautiful women”. Plath, whose dynamism conceals a dangerous emotional fragility (she narrowly survived a suicide attempt at 20) is thrown into crisis.

In February 1963, having moved to London for the winter, she killed herself in her rented flat. She left behind the manuscript of Ariel – one of the most incandescent cries of rage and defiance in English literature, written during the “agonised and degraded” months after she discovered Hughes’s infidelity. It comprises, as she recognises before her death, “the best poems of my life”: poems that would, on their eventual publication in 1965 by her stunned widower, make her name.

Why not content ourselves with the poetry and avoid eavesdropping on the humiliations of Plath’s short life, as she recounts them in these letters? Simply because she, more brilliantly than anyone, demonstrates what TS Eliot calls the poet’s ability to form “new wholes” out of chaotic and disparate experiences. Peter Steinberg and Karen Kukil have produced the first complete text of all Plath’s known correspondence: every extant readable word she wrote to her needy mother as well as previously unpublished letters to Hughes’s family and her own friends and professional mentors in literary London and America, some 1,400 letters in all. And what a tour de force it is. Plath’s epistolary style, as the editors suggest, is “as vivid, powerful, and complex as her poetry, prose and journal writing”. Her energy even when she is doing or observing the most ordinary things vaults off the page. She paints floors, sews curtains, learns Italian and speedwriting, works part-time to “free Ted from a dull job to support us”, answers his fan mail, keeps the household accounts, mows the lawn. She has an uncanny ability to make the mundane strange, whether it is a room painted “the yellow shade of spoiled pears”, a ceiling flaking “like leprosy”, or a terraced house “nightmarishly exactly like all the rest, except that it has a small, tortuously withered tree by the front hedge”. As a citizen of the “land of milk and honey and spin dryers”, never wholly at ease in grim postwar Britain, she is mesmerised by the cold and dirt, the “filthy, cheerless, lightless, bathless places” for rent, the diminutive fridges, the scrawny chickens in the butchers’ windows.

But Plath’s panic is, at bottom, existential. There is a terror in her of being alive at all, which gives her poetry its hellish edge but which the letters keep valiantly at bay. It’s through the material world, and her own body in particular, that she makes herself feel real: through food and the touch of the sun on her skin; sex and childbirth. Almost every letter contains a sacramental parsing of a meal she has cooked or eaten – nobody apotheosises “stuffed tomatoes, turkey, lemon mousse & Chablis” like Plath – while her ecstatic accounts of bearing and nursing her two children are unequalled anywhere. Hughes, meanwhile, is not just her husband; he is her religion. “My marriage is the center of my being,” she admits, “I have given everything to it without reserve.”

Through her body she makes herself feel real: through food and the touch of the sun on her skin; sex and childbirth

This is why his betrayal breaks her. At the radioactive core of this volume are 14 letters that Plath wrote to her psychiatrist in America, Dr Ruth Beuscher, the contents of which – among them a sensational claim by Plath that Hughes beat her up shortly before a miscarriage – were selectively leaked to the press in 2017 when the bundle was put up for sale by a private owner. On being bought by Smith College they were passed on to Frieda Hughes, the literary executor of Plath’s estate. In deciding whether or not to publish these letters she is in the impossible situation of a child who finds herself being asked to arbitrate between warring parents. To her credit she allows them to be included, and lets us make up our own minds.

The greatest surprise of the Beuscher letters, which are far franker than those Plath writes to anyone else, is the initial steadiness with which Plath assesses her situation after Hughes announces that she has been “a jinx, a chain” and that he now wants “to experience everybody & everything”. In contrast to the myth – promoted by Hughes in Birthday Letters (1998) – that his infidelity was somehow predestined, she insists wryly that in marriage “there is some purpose … in riding through infatuations without always indulging yourself, if you know it hurts someone”. Her husband, she realises, has carried out the cost-benefit analysis of an affair that he suspects will be psychic death to her and has decided that it is worth it. “Do not imagine that your whole being hangs on this one man,” Beuscher counsels her, but that is precisely what Plath does feel. Her gradual disintegration, “the return of my madness, my paralysis”, is dreadful to watch. Though Hughes later claimed that she didn’t seriously consider divorce, this is belied by her desperation to “become a verb, instead of an adjective”; to claw back her independent self and “never bind it to anyone again”. Too late. During her final mental collapse she sees very clearly that her readiness to predicate her identity on her marriage, as she puts it in one of her last poems, “Death & Co.”, has “done for” her.

Even in extremis, Plath’s wit doesn’t desert her (Wevill is “this Weavy Asshole”; when Plath asks for a copy of a friend’s crime novel, she jokes that “it looks just the thing to cheer me up, all about murder”). Nor does she lose her poetic judgment. She finds a stash of love letters written by Hughes to Wevill, “describing their orgasms, her ivory body, her smell, her beauty”, and though she is “just dying” she concedes that these are “fine poems”. What about her grasp of the truth? Did Hughes really “beat” her? There is evidence scattered throughout Plath’s letters and diaries that their arguments could become physical, and that she was at times not just the aggressor, but gave as good as she got. From her 10th letter to Beuscher it’s apparent that they fought in this way again one day in early February 1961, after she had torn up some of Hughes’s papers to spite him when he made her late for work. The difference is that on this occasion Plath was four months pregnant. Exactly how much force was involved, and whether the miscarriage that followed was linked to this fight, or to a grumbling appendix – as Plath herself believed back then – or to neither thing, is anybody’s guess. It doesn’t need saying that pushing your pregnant wife around is inexcusable.

In the aftermath of a disaster, it is the survivors who get to tell how things happened. For the moment though, Plath, who writes in the dark days before her death that “it is my great consolation just now, to speak & be heard”, has the last word.

• The Letter of Sylvia Plath: Letters Vol 2 by Sylvia Plath (Faber, £35). To order a copy for £30.10, 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.