These letters between Vera Brittain and Winifred Holtby cover 15 years of a remarkable friendship that began at Somerville College, Oxford in 1919 and ended only with Holtby’s premature death from kidney failure in 1935.

Brittain went up to Oxford in 1914, but left to serve as a nurse in the first world war. She returned freighted with tragic experience, having lost both her lover and her brother and tended the wounds of horribly injured soldiers close to the front. She disconcerted younger undergraduates with her fiercely competitive and forthright views combined with fragile looks and a general air of suppressed trauma. Holtby, five years her junior, had also interrupted her studies to serve as a nurse for a year, but, as a big, clumsy, friendly giant, popular throughout the college, she could not have been more different. After an initial stand off, the two tutorial partners – both reading history – became inseparable, with the giant taking the nervous wreck under her wing and then, quite quickly and for the rest of time, sitting at her feet.

They lived together after Oxford, first renting a studio in Doughty Street, London, where they settled down to writing novels in between travelling the country as lecturers for the League of Nations and suggesting articles for Time & Tide, the feminist magazine founded and funded by Lady Rhonnda – who went on to make a great pet of Holtby but not of Brittain. ‘As you know, you are always extremely attractive to women,’ Brittain wrote matter of factly to Holtby. She herself was happier in the company of men; yet Holtby always maintained that she learnt her feminism from Brittain, whose conventional provincial background had rendered her gritty with grievance.

Holtby had been propelled into Oxford by her strong-minded Tory mother – the model for Mrs Beddoes in South Riding, Holtby’s posthumously published novel, set in Yorkshire – who sat on the county council and later served as an alderman. She was convinced that nothing but the best would do for her gawky, bookish daughter. Holtby also maintained that Brittain was the better writer with the finer mind. For much of this selected correspondence, Brittain appears to accept Holtby’s reassurances as her due, but sometimes she fails better. Here she is in 1922, writing of Holtby’s first novel, Anderby Wold, which had been accepted by a publisher while her own (which just so happened to contain a cruel caricature of Holtby) had been turned down: ‘I think I pretended that it bored me mainly because it gave me a despairing sense of my own inability to reach the same level.’ There is no whisper of the pretence of boredom in Holtby’s side of the correspondence, which invites one to wonder what other cruelties were sometimes visited on her by the friend whom she addressed, and who often signed herself, as ‘v.s.v.d.l’ – ‘very sweet very dear love’.

In 1925 Brittain married an English political economist with a pince-nez called Gordon Caitlin, after a courtship conducted mainly by letter. ‘It seems that one must choose between stagnation and agitation in this world,’ wrote Holtby mournfully when Caitlin first appeared on the scene. She too had a sort of suitor, who seems to have suited her by existing but did not suit her as much as Brittain did. Meanwhile, Brittain wrote from her honeymoon in Austria that she ‘would not sacrifice one successful article for a night of physical relationship’ and advised Holtby not to think of getting married herself:

One night of what lawyers call cohabitation would be enough for you. Please can we go away as soon as I come back. I need you more than I need him because I care about writing much more than I care about being a wife.

After a year in America, Brittain returned to London and the flat she had previously shared with Holtby. A disgruntled Caitlin came and went. In 1927 she gave birth to a son, and three years later the household moved to a largish house in Glebe Place where her daughter Shirley, later Williams, was born. ‘Look! she’s got my hair,’ Holtby would say, holding the baby. This story appears in Paul Berry and Mark Bostridge’s life of Brittain. I mention it because that tone, jokey and unafraid, is mainly absent from the letters.

In fact it is notable that while this volume offers a window on an intriguing relationship, neither writer is shown to much advantage in the correspondence, which they picked up whenever they were apart. In the early 1930s Holtby was often away, visiting her family in Yorkshire, or recuperating from the bouts of illness which dogged her, or holed up in cottages on the north coast, rushing to finish South Riding before the death she knew was coming but to which she never referred.

Meanwhile, Brittain was working hard on the war memoir which would make her famous. But you would not guess the power of Testament of Youth from her anxiously circling screeds during the years of its composition. The juxtaposition of her youthful ambition and self-absorption with the unfolding horrors of a war which she had been proud to see her brother and his friends join with such confidence and high spirits make her book compelling. When the ambition and self-absorption outlive the backdrop of world war, that grim tension goes missing.

Holtby reiterates her faith in Brittain’s project and rarely refers to her own plots or themes, as she develops the satiric gift which still glitters in enchanting novels such as The Land of Green Ginger (1927), which tells the story that the letters deny of the limits of friendship, or Poor Caroline (1931), a playful reworking of the tale of a pitiable spinster. The panoramic energy of South Riding is a remarkable achievement, written against the odds, but it is a less surprising novel than some of the earlier ones.

Holtby, then, was the darker horse with the lighter touch; but her letters to Brittain are for the most part too doggedly responsive to allow for mystery or wit. The exceptions are those she wrote from South Africa, where she went in 1926 when Brittain was in America, reluctantly helping Caitlin with his research work while plotting her permanent return to England with the baby she was expecting. These dispatches take on a different tempo, as the delay between posts frees Holtby to try her hand at some lively sketches. Brittain didn’t like this at all, and when Holtby returned to England it was to a series of increasingly paranoid letters accusing her of indifference and worse. Holtby was reduced at once to her usual state of abjection.

The editors of this volume, surveying the ways in which the overworked Holtby came to Brittain’s rescue in managing her husband, her children, her parents and her books, are in no doubt that ‘Vera was the chief beneficiary of the arrangement’. They also quote the novelist Stella Benson,

who told Winifred to her face that Vera was her ‘bloodsucking friend’. But Winifred was unperturbed. Perhaps, Benson noted, Winifred ‘liked having her blood sucked’.

I’m team Benson. There isn’t a single instance in these letters of Holtby failing in her loyalty to Brittain, and her declarations – of devotion, of pleasure in service, of gratitude that she was allowed to serve – have the inarticulate ring of authenticity. Moreover, there was some give and take in the way they finished one another’s articles and corrected one another’s proofs. Indeed, everything that Holtby learnt from loving Vera, looking after her children and living at such close quarters to a marriage which satisfied neither party went into the fiction, which she was still writing the week before her death. The world saw her being fagged, but she knew better. ‘You are the only person who doesn’t stop me working,’ she wrote to Brittain from her sickbed. It was the highest praise.

And Brittain served her handsomely in the end. She saw South Riding into print in the teeth of Mrs Beddoes’s late-onset humour failure. She followed that up with Testament of Friendship (1940), the sort of book she wrote best – one about herself in which a beloved dead person is brought vividly to life. Twenty years ago, Marion Shaw’s excellent biography of Holtby explored other relationships, to present a portrait subtly different from the one of Brittain’s Testament. Between Friends delivers Holtby back into Brittain’s ineluctable embrace.