When the first volume of The Letters of Sylvia Plath appeared last September, its chief virtue – its only virtue, in my view – was that tucked into its massive bulk were the 16 love letters Plath wrote to Ted Hughes in 1956, the year of their marriage. These notes, so marvellously lucent, did not make the rest of the project – in essence, the publication of dozens upon dozens of quotidian and repetitive letters to Plath’s mother, Aurelia – much more worthwhile; at more than 1,400 pages, the collection as a whole seemed to me to be of interest only to desperate PhD students and snouty scholars. But still, they were quite something, their radiance emitting just a little of Plath’s talents as a poet; her awe that such a man as Hughes could exist at all – parted from him, she would crumple like paper – hinting at the trouble that would lie ahead once they were wed and claustrophobia had set in.

Those letters were published thanks to the agreement of their owner, the couple’s daughter, Frieda Hughes, whose equanimity and clear-sightedness in the matter of her parents has long been a remarkable and precious thing. Somehow, Frieda has managed – outwardly, at least – not only to live with the fact that the nature of their relationship is contested by strangers on an almost daily basis, but to continue to hope that those participating in this horrible tribal war can somehow be made to see that there are no saints here, just as there are no sinners; that both her parents were flawed; that both of them, in the months and weeks before her mother’s suicide in 1963, were suffering in a similar, if not precisely the same, kind of hell.

Now, with the arrival of volume two, she rides to the rescue once again, having decided that the 14 letters her mother wrote to her psychiatrist, Dr Ruth Beuscher, between 1960 and 1963 should be included in its pages after all (these letters, the existence of which she did not know until 2016, were controversially put up for sale by a book dealer in the US last year; as a result, several newspapers were able to quote from them; they’re now in the possession of Plath’s alma mater, Smith College, Massachusetts). As she notes in her foreword to the book, ultimately she could not bear the idea that this correspondence, however raw and private, would remain buried in a library, for all that such an option was in her power. The thought that the sensationalist phrases reported out of context by the press would go unchallenged in the public memory left her feeling “suffocated” – for which reason she found herself ringing Faber, her mother’s publisher, at the very last moment (“there was just time”, she writes, relievedly).

These letters don’t make volume two, which covers the period from Plath’s marriage until her death, worth your time in its entirety. Running to more than 1,000 pages, it comes with all the same problems as its predecessor. The vast majority of the letters are again to Aurelia Schober Plath, are concerned mostly with homemaking (after a period spent living and working in the US and London, Plath and Hughes moved to Devon, where she gave birth to her second child, Nicholas) and are written in her brightest, most determined voice – the one that brooks no argument, that allows for no hint of a shadow. Again, the editors, Peter K Steinberg and Karen V Kukil, identify every letter according to which institution holds it, rather than by where it was written, which makes life laborious for the reader. But Frieda was right: the letters to Beuscher are extraordinary, throwing vital light on Plath’s mental state in the period after she discovered Hughes’s affair with Assia Wevill.

Amazing as this may sound, you half wonder if she isn’t going to make it, to build the new life of which she dreams. Like someone on a cliff face, her hands reach for every hold; inch by inch she pulls herself up. “I felt the most fantastic exhilaration,” she writes in October 1962, Hughes having left for good. “After I drove him to the station with his things, I returned to the empty house expecting to be morbid… [but] I was ecstatic.” Only at the last moment does she falter, the tips of her fingers unaccountably losing their grip.

In these 14 letters (including the last letter she ever wrote, dated 4 February 1963), Plath’s tone is unbridled, threaded with the “incandescent desperation” (whether she is feeling happy or sad) that Doris Lessing recognised in her on the only occasion they met. She writes of sex, and how much pleasure it gives her (“I do not want to be the un-fucked wife,” she says, pondering aloud the question of whether she can continue to be married to Hughes even as he takes a mistress), and of her fear that she will now become her mother, descending into bitterness (“right now, I never want to see her again”). But though we know how this would end for her, such things speak only of the ordinary madness that descends on any lover left broken-hearted. How much more stirring is the fact that on having found the “sheafs of passionate love poems” Hughes has written to Wevill – poems that describe “their orgasms, her ivory body, her smell, her beauty” – she is still able to see their quality. “Many are fine poems,” she writes. There is a writerly nobility here that brought me almost to tears.

In death, Plath’s legacy has often been the victim of simplified feminist ideology. Her letters, however, complicate this by being, almost until the end, unstinting when it comes to her passionate belief that “marriage is the central experience of my life… much more crucial than a religion or career or anything”. She is not what you would call sisterly; the greatest insult she is able to throw at a woman is barrenness, the fact that she has had multiple abortions (I wonder, too, what readers will make of her repeated references to the “Yorkshire-Jew minds” of the supposedly parsimonious Hughes family; to her professed distaste for the dirty, lazy English working classes).

She is convinced that Hughes is a genius, and feeds him roast beef for breakfast, the better that his manly brain might work its uncommon magic. She is obsessed with money. She is needy and demanding, suggestible and narcissistic. I should say that none of this remotely bothers me; I do not expect, or even much want, my literary heroes and heroines to be nice. Their imperfections and limitations only make them the more human. But it will trouble some, and over the course of so long a book, Plath’s voice, hectoring and frequently manipulative, is undoubtedly wearying. Reading a clutch of her most dazzling poems once I’d put it down was like flinging open a window. After the stale air, a breeze so sharp it fairly stung my eyes.

• The Letters of Sylvia Plath Volume II: 1956-1963, edited by Peter K Steinberg and Karen V Kukil, 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