Sutton Publishing £20 pp288

Anna Sewell wrote her first and only book when she was 57. An obscure, pious spinster and chronic invalid, she did not look set for bestsellerdom, and London booksellers bought just 100 copies on publication. But the book was Black Beauty. In the next 15 years it sold 1m copies, and worldwide sales passed the 40m mark in the 1990s.

Writing her biography is a challenge, because so little is known. She was just a loved daughter, sister and aunt, with no prospect of fame, so nobody bothered to interview her or keep any records. Apart from the bare outline of her life, a few letters and scraps of a journal are all that survive. Her family were Norfolk Quakers. Isaac, her father, tried many vocations, from drapery to bank-managing, and was not much good at any, so they kept having to move house, and money was often short. In 1834, when Anna was 14, she slipped and twisted her ankle running home from school. It refused to heal, and precipitated a mysterious illness which lasted all her life. Adrienne Gavin thinks it was a severe form of lupus, an auto-immune disease in which the body attacks its own organs. There were periods of remission, but often Anna could not walk and suffered acute pain. Pills, potions, homoeopathy, water-cures and visits to foreign spas all proved unavailing. Only horse-riding helped, giving her back the freedom her body had lost. This was the start of her close identification with horses. Black Beauty’s fall, which breaks his knees and ruins his prospects, mirrors Anna’s.

Because the record is sparse, every scrap must be squeezed to release its significance, and Gavin is a good squeezer. By the time she is through, it seems inevitable Anna should have written just the book she did. Her mother Mary was the cornerstone. An independent, strong-minded woman, she was a schoolteacher when she married, and made herself responsible for educating Anna and her brother Philip. For the period, her teaching was unusually rational and scientific. The children did chemical experiments, and learned to handle spiders and beetles with pleasure, laughing, Mary recalls, at those who feared them. Cruelty, they were taught, was abhorrent — “the devil’s trademark”, as John Manly calls it in Black Beauty. They were never punished. Being denied a kiss was enough to bring on tears of repentance, Mary said. That lesson, too, was not lost on Black Beauty’s author.

In the Quaker tradition, Mary was a dynamo of good works. Wherever the family moved to, she organised soup kitchens, childcare classes, lending libraries and prison visits. As soon as she was old enough, Anna joined in. In a village near Bristol, where they settled when she was in her 30s, she ran evening classes for “men and lads”, teaching geography and natural history. She would astonish her students by dissecting a sheep’s or bullock’s eye collected from the butcher, and explaining its anatomy. From her contact with the poor she learned that the purpose of life is to alleviate suffering, which is the moral of Black Beauty. As important, she learned to talk to ordinary working people. She was not “one of your stuck up ones”, a workmate of Philip’s attested. In a journal fragment, she records a long, serious conversation with a cabman. The knowledge of ostlers, farriers and grooms that surprised early readers of Black Beauty was the fruit of her sociability. The chapter where the old war horse Captain describes the charge of the Light Brigade seems to reflect contacts she made when the family was living near a cavalry barracks in Chichester at the time of the Crimean war.

Mary loved literature, and encouraged Anna’s reading. They learned and recited great swathes of poetry together, especially the Romantics, and Isaac’s poor performance as a breadwinner impelled Mary to try authorship herself. Her first book was a natural history primer for toddlers. But sentimental ballads were her true métier. The most famous, Mother’s Last Words, a heart-rending tale of two orphan boys, sold a million copies. It was valued in prisons and reformatories, where readings of it reduced hardened criminals to tears. All Mary’s compositions were tried out on Anna first, and she became an expert critic. Her niece records that in the 1840s Anna met Tennyson at a spa, where they were staying at the same hotel. They had “walks and talks” together, and he gave her a signed portrait of himself. That she made such progress with the shy, difficult young Tennyson indicates what a literary person Mary had turned her into.

Gavin does not speculate about the origin of Anna’s prose style. But the mention of her reading Bunyan’s Holy War surely gives a clue. Black Beauty is in the tradition of plain, clear Puritan prose that raised Bunyan to classic status. The unforgettable moment in Pilgrim’s Progress, when the character called Ignorance is thrown down to Hell from the very gates of Heaven, seems to be picked up in Black Beauty when Manly rails against Joe the stable-boy, who has ignorantly given the horse cold water after a gallop, almost killing him. “Only ignorance! How can you talk about only ignorance! Don’t you know that it is the worst thing in the world next to wickedness?”

This uncompromising verdict suggests why Anna’s niece thought her strict. But she was severe with herself too. Illness had made her question God’s justice, and she went through a period of depression and religious doubt. But she steeled herself, and emerged triumphant. “I thank God for my lameness,” she wrote, “I am sure it is sent in love, though it be a trial.” Her stoicism gave her fellow feeling with horses, who, as Black Beauty remarks, “are used to bearing their pain in silence”. She made a habit of talking to them, her niece remembers, appealing, when necessary, to their better nature, and using the Quaker form of address: “Now thee must go a little faster — thee would be sorry for us to be late at the station.” It was only a step from this to the horses talking back, as they do in Black Beauty. Its title page said it was “Translated from the original Equine ”.

Thousands of copies were distributed free to stable-hands and drivers by animal welfare groups, and it was translated into Arabic, Chinese, Japanese, Hindustani, Turkish and Braille, as well as the European languages. Like Uncle Tom’s Cabin, to which it was often compared, it is one of the few novels that can claim to have reduced suffering. That is what Anna wanted. It was her last blow against cruelty. She started writing it because the doctors had given her 18 months to live, and she died four months after it was published. Gavin’s biography makes it seem an even finer book than before, which is what all literary biographies should do — and would, if sensible, high-principled creatures like Black Beauty wrote them.

Available at the Books First price of £16 plus £2.25 p&p on 0870 165 8585

Explore Books

• Book Extracts

• Book Reviews

• Books Group

• Audio Books

Times Recommends

• Superfreakonomics by Steven D Levitt and Stephen J Dubner
• The Times picks the 50 best paperbacks of 2009
• Vincent van Gogh’s letters show the soul of the artist