In early 1956, Sylvia Plath wrote a long, digressive letter to a man she thought she loved, during which she sarcastically demanded, “How symbolic can we get?”, mocking her own youthful desires. Within a few weeks, she would meet Ted Hughes, and the story of the two poets’ love affair and its tragic aftermath has left readers all over the world identifying with their story, endlessly amazed at how symbolic it could get.

After six years of marriage, two children and one miscarriage, Plath discovered that Hughes was having an affair. As they were moving painfully towards a breakup she wrote a poem called Burning the Letters: “I made a fire,” she said, “being tired / Of the white fists of old / Letters and their death rattle.” These letters taunted her, she implied: “What did they know that I didn’t?”

Now, almost 55 years later, we hear once more the death rattle of letters that know something we don’t about Plath and Hughes – in this case, letters Plath wrote to her friend and doctor Ruth Barnhouse, apparently near the end of her life.

Part of what makes the story of Plath and Hughes feel so endlessly symbolic is the way it functions as a lightning rod for changing ideas about sexual politics. Letters are almost as central to the cultural story of Plath and Hughes as the poems they wrote to, and about, each other. Twenty years ago, I embarked on a PhD about Plath’s poetry, but became so fascinated by the tug-of-war over her legacy, our culture’s long and determined effort to turn her from an author into a character that I ended up writing a thesis about that story. Now new documents emerge, and the whole tale gets retold – once more, with different feelings.

The facts may alter with new evidence, but mostly it’s our interpretations that have altered. Our ideas – about feminism, marriage, mental illness, suicide and domestic violence – change and with them our attitudes towards Plath and Hughes.

Sympathies shift, from her to him and back again. Just as I was finishing my thesis came the news that Hughes had suddenly published a collection of poems, Birthday Letters, followed soon by Howls & Whispers, an addendum of sorts. One of the many people at whom the dying Hughes took bitter aim in these final poems, accusing them of being the true authors of Plath’s suicide, was Barnhouse.

Barnhouse had been Plath’s psychiatrist since she was taken to hospital in 1953 after a breakdown and suicide attempt in her early 20s; the experience later inspired Plath’s only novel, The Bell Jar. Barnhouse remained Plath’s therapist, treating her during the periods that Plath lived in the Boston area, and corresponding with her as a friend when she lived in England, including her final sojourn after she returned with Hughes. Barnhouse later said that she had burned her letters from Plath, while Hughes sealed the letters from Barnhouse in the Plath archives at Smith College, where they will not be fully available for years.

But now some of the letters from Plath to Barnhouse during the last months of Plath’s life have surfaced. Given the intimacy and candour of their relationship, and given that Plath’s final journals were lost and destroyed (by Hughes, he said), these letters are set to become one of the only sources of Plath’s voice we may have from the end of her life, apart from her poetry. It seems she told Barnhouse that Hughes had been violent, and accused him of causing the miscarriage of their second child – a traumatic episode, as is clear from more than one poem.

Although the question of violence in their marriage is nothing new, a direct accusation by Plath that Hughes caused her miscarriage certainly would be. From the famous moment when they met at a Cambridge party as ambitious young poets, and came together with an erotic force that was charged with violence: he grabbed her headband from her head, and she bit his cheek, she claimed in her journal, until it bled.

The writing of both poets overflows with violent images, of which letter-burning is probably the most innocuous example. Both she and Hughes were scenery-chewers, people who saw themselves as larger than life.

Plath mythologised marriage and procreation as the source of her inspiration; Hughes concurred, and claimed the primal urges of hunting and art were at one. The legend, in other words, was produced by Plath and Hughes themselves, and violence was at its heart, a violence that was never merely symbolic. Her death made that all too clear.

Everything ended in a rage, and in the determination of tragedy. Anger – as it tends to – left everyone polarised. Love was thwarted, and perverted; sorrow was appropriated and refashioned; their story was used to act out a cultural drama which was only partly about the battle of the sexes. It was also about our desire to know, a desire that writers always invoke and then frustrate. In Burning the Letters Plath warned: “Only they have nothing to say to anybody. I have seen to that.” How symbolic can we get.

• The ninth paragraph of this article was amended on 12 April 2017. It originally stated that the allegation that Hughes may have caused Plath’s miscarriage was new. This has been suggested before, but a direct accusation by Plath that Hughes caused her miscarriage would be new.