ANNA Sewell (1820-1878), creator of the most famous black horse ever, is herself the ultimate dark horse.

During her life no one suspected that in her final years she would write a best-selling novel that would still be published, read and loved more than 125 years later. Neither was she lionized after her death, perhaps because Black Beauty was her only book, or because she died mere months after its publication. While her novel lives on as the horse book above all others, Sewell herself has until now all but vanished from literary history.

Yet claims have been made that Black Beauty (first published in 1877) is among the top 10 best-selling novels ever written. In 1923 a critic could write: "than Black Beauty only the Bible has found a wider distribution," and in 1995 its worldwide sales were estimated at 40 million, compared to 50 million for Dickens's complete works. Lesser claims would warrant more interest. Yet surprisingly Dark Horse is only the second full-length biography of Sewell and the first for over 30 years.

Making use of numerous recently discovered letters by Sewell and her family, Adrienne E Gavin offers new insight into the life of this enigmatic woman.

Lamed for life at 14, expelled from the Quakers at 18, teacher, editor, painter, and passionate animal lover, Sewell also suffered from mysterious and ongoing illness. Her physical restrictions making her more than usually dependent on horses, she became an excellent horsewoman with an intuitive equine understanding. She spent years living in a string of towns across Southern England: Great Yarmouth, London, Brighton, Lancing, Haywards Heath, Chichester, Abson in Gloucestershire, Bath and Old Catton near Norwich. Her local connections are best known in Norfolk, the county of her birth, death and family heritage, but people in most of the locales she lived in claim that the original of Black Beauty once grazed in nearby fields.

The great are often written about, but not always the good. Sewell strove successfully to live a life of practical altruism. A thinker of tenacious spirit and physical courage, she lived up to her own ideals and achieved, against painful odds, something extraordinary. She created that rarest of things: a book which not only continues to be published, bought and read, but which also continues to be loved. Not written as a children's novel, Black Beauty has shown itself to be a book of multiple capacities: horse-care manual, passionate protest against cruelty, novel of Victorian life and attitudes and, how we know it best today, a beloved classic.

This new and compelling biography reveals the life of Anna Sewell: her experiences, her beliefs, and her sympathy with animals and their suffering that inspired her famous work.

Dark Horse: A Life of Anna Sewell by Adrienne E Gavin (Sutton, £20)