George Orwell’s enigmatic first wife was, it appears, an exact inversion of his second wife. Eileen O’Shaugh-nessy married an unknown writer and then worked herself to death to ensure his fame, while Sonia Brownell married a dying writer in order to ensure her own fame. The names say it all: Orwell was christened Eric Blair and Eileen therefore called herself Eileen Blair; their son, Richard, was also Blair. Sonia, however, who married Orwell on his deathbed, adopted his nom de plume and kept it even after she remarried.

We know a lot about Sonia and nothing about Eileen. Because the correspondence between herself and Orwell has not survived, Eileen has become what DJ Taylor calls “something of a black hole at the centre of Orwell studies”. Orwell’s friends found her defensive and elusive, Eileen’s friends thought her playful, untidy, and a tad malicious, and Sylvia Topp – Eileen’s first biographer – concludes from her subject’s handwriting that “she had a need for attention and sometimes put an act on to impress others with her intellect”. Eileen, however, described herself as “very much like Eric in temperament which is an asset once one has accepted the fact”. Their shared temperament might be described as stoical and combative, with a turbo-charged death-drive.

Letters discovered in 2005 to a college friend, Norah Myles, confirm two things about Eileen: that she might have been, as everyone agreed, a writer herself, and that her character was completely unreadable. In the first of the letters, sent five months after her marriage in June 1936, Eileen apologises for not writing sooner, but she and Eric have “quarrelled so continuously & really bitterly that I thought I’d save time & just write one letter to everyone when the murder or separation had been accomplished”. To what extent was she joking? Could even Norah tell?

In another letter, Eileen describes the Blair family’s response to their marriage: “they all adore Eric & consider him quite impossible to live with – indeed on the wedding day Mrs Blair shook her head & said that I’d be a brave girl if I knew what I was in for, and Avril the sister said obviously I didn’t know what I was in for or I shouldn’t be there”.

Whether she did know is a moot point. On the one hand, Topp stresses that Eileen was “not willing to be a subservient wife” and Orwell was uninterested in subservience, and on the other she argues that Eileen chose “to abandon her personal pleasures” in order to live Orwell’s life to the full, a sacrifice he took completely for granted. Regardless of whether she was “the making of George Orwell”, as Topp’s bold subtitle claims, we are left with the strong impression that Orwell was the breaking of Eileen.

Eileen O’Shaughnessy was 30 when she married Orwell in 1936, and 39 when she died. Dark-haired, blue-eyed, with a cat-shaped face and pale Irish skin, she read English at Oxford and was “bitterly disappointed” to not get a First. Giving up her dream of academia, she set up her own typing agency and then began an MA in psychology at University College London. It was then that she met Orwell at a party.

Two years older than Eileen, Orwell was a 6ft 4in maverick with a “moth-eaten” appearance and deep lines on his face. During their nine-month courtship he worked on Keep the Aspidistra Flying (1936) and The Road to Wigan Pier (1937), while Eileen gave up her MA and all human comforts to live with him in a tiny 300-year-old cottage in Wallington, Hertfordshire, with a 4ft 6in-high front door, chronic damp, no electricity, one cold tap, an outside privy and a cesspool that became blocked when the wrong toilet paper was used. They got a goat to milk and some chickens for eggs and ran a grocery shop from the front room.

When Orwell, who bent himself in half to get through the door, used the paraffin light for his writing, Eileen had to type his manuscripts in the dark. The kitchen flooded, the food went mouldy and Orwell, in denial about his tuberculosis, was permanently ill. Eileen confided to Norah that she “cried all the time from pure exhaustion & partly because Eric had decided that he mustn’t let his work be interrupted & complained bitterly when we’d been married a week that he’d only done two good days’ work out of seven”. These, Topp says, were the happiest months of their marriage.

The honeymoon over, Orwell went to fight in the Spanish Civil War, where Eileen joined him and had an affair with his commander, Georges Kopp. The couple celebrated their first anniversary in hospital, where Orwell was recovering from a gunshot wound through the throat.

A central source of information about Eileen is her best friend Lydia Jackson, who claims that she was “not happy in that marriage from the very beginning”. But Lydia, who had an affair with Orwell when he returned from Spain and was writing – with Eileen’s help – Homage to Catalonia (1938), is an unreliable narrator. What was going on with these affairs? Topp vaguely explains that “Orwell and Eileen seem to have agreed on a somewhat open marriage”, the words “seem” and “somewhat” need further clarification, particularly given that Orwell’s affair with Lydia was conducted in secrecy.

In the second year of their marriage Orwell suffered a haemorrhage and went to recuperate in a sanatorium. Here he wrote book reviews and planned Coming Up for Air (1939), while Eileen, whippet-thin and clearly unwell, milked the goat, fed the chickens, ran the shop, typed the manuscripts, and made, every fortnight, the ten-hour round trip to see her husband. Having narrowly survived the Blitz, she reluctantly agreed to adopt the baby boy Orwell had longed for. Their son was a year old when Eileen died while undergoing a hysterectomy. Orwell, meanwhile, was working in Paris.

This biography is meticulously researched. Like a mathematician, Topp shows her workings and thus loads her pages with evidence of her time in the archives. For example, we get the transcript of every school report and every tutor report for all three years of Eileen’s English degree at St Hugh’s College (“Miss O’Shaughnessy has a sense of style and when she translates correctly her work is quite good” etc). It’s fun for the biographer to discover these titbits but they are less fun to read, and should be condensed or confined to the endnotes rather than displayed like a centrepiece.

Three anonymous articles in the London Evening News, that may or may not have been written by Eileen, are similarly quoted in full; we are given the exact wording of Eileen’s father’s will and copies of his correspondence as collector and surveyor of customs in South Shields. “If it is possible to cut a word out,” Orwell counselled, “always cut it out”: according to his rule, entire pages of Topp’s book should be excised.

The deluge of detail mars an otherwise moving and important book, and obscures the central question: what did Eileen Blair want? The sphinx has kept her secrets.

Frances Wilson’s books include “Guilty Thing: A Life of Thomas De Quincey”