Josephine Pullein-Thompson: Horsewoman whose jolly pony books brought pleasure to generations of girls and who later served English PEN

The oldest of three sisters who all wrote best-selling pony books for children, Josephine Pullein-Thompson, like the rest of her family, drew her stories from an unorthodox but very active childhood and youth. Dying aged 90, she leaves behind her sister Diana, whose twin Christine died in 2005. All her novels were set in a world of traditional countryside practices and pursuits common enough when she was young but remote today.

Born in 1924, she was the daughter of Joanna Cannan, herself a considerable author whose novel A Pony for Jean (1936) broke new bounds by concentrating not so much on the steeds but on their young riders. Her father, Captain Harold Pullein-Thompson, was a well-born former schoolteacher in constant pain fromFirst World War injuries. Hot-tempered, given to knocking off the hats of those still wearing them during performances of the National Anthem, he spent more than he earned, leaving the family constantly hard-up.

Small and light as a child, Josephine often felt excluded by the identical twins born 18 months after her who spoke incomprehensible twin-talk to each other for their first five years. But she adored The Grove, the family home in the South Oxfordshire village of Peppard near Henley-on-Thames, which the family moved to in 1930. This former dower-house with its own spinney and five acres of land, was ever after shared with dogs, cats, bantams and ponies.

Possessing only one set of riding clothes to be shared between all three, the sisters soon competed vigorously in local hunts and horse shows, their names when announced often greeted with groans from other contestants, given their growing record of winning prizes. Hating their various schools, which they attended part-time, as their mother preferred having them at home, they read voraciously and were banned from all equine talk except at tea for fear of becoming "horsey bores".

Initially wanting to be a vet but hampered by her lack of education, at 16 Josephine decided instead to become an equitation expert. From 1939 the girls had been running their own increasingly successful Grove Riding School, specialising in taking on difficult horses, which more often than not they were able to reform.

In 1946 she and her sisters had their joint story It Began with Picotee published, written around the table five years earlier. In the same year Josephine produced her first solo effort, Six Ponies, written while she was on the roof of the Reading telephone exchange while briefly working as a telephone engineer. Many more titles were to follow, with the girls making enough money from writing to be able to give up the riding school altogether.

Composed at speed, with little attention to the niceties of style or language, Josephine's subsequent 32 pony books were popular for the next two decades but eventually seemed dated, with characters madly dashing through the pages, sometimes confessing to being awfully or frightfully silly while remaining jolly well determined to win at the next gymkhana before celebrating with a supersonic tea-party.

But beyond their restricted vocabulary and formulaic Cinderella, Ugly Duckling or rags-to-riches plots, there were some excellent set pieces describing everyday countryside alarms such as a bull escaping or a fire breaking out near a hay barn. There were also examples of dry wit, with a horse described as "looking like a trade unionist who'd been standing up for his rights and got them." Each book contained sound technical advice hard-won over the years and augmented by the author's voluntary work as a Pony Club district commissioner. Her last novel, A Job with Horses, appeared in 1994.

Also writing a couple of adult detective stories plus five non-fiction equestrian titles, Pullein-Thompson became closely involved with the English Centre of International PEN in the 1970s, becoming its general secretary and finally president. Determined, if sometimes abrasive, she strove to improve the lot of writers, particularly those in oppressive regimes. Never marrying, she was later the partner of Anthony Babington, a circuit judge paralysed down his right side due to Second World War head injuries. Always in danger of falling over, he was cared for by Josephine for over 20 years with all the commitment she had shown to her own lamed father as well as to countless injured or neglected animal pets. He died in 2004.

Awarded an MBE in 1984 for services to literature, Pullein-Thompson lived on quietly in a small terraced house in Fulham, befriending the blackbirds in her garden and keeping in touch with her nine nieces and nephews. In 1996 she and her sisters wrote Fair Girls and Grey Horses: Memories of a Country Childhood, an illustrated account of the lost world of their childhood. This enchanting memoir is to be republished this summer by Alison & Busby.

In her own conclusion to it, she writes that because of their rich, if often tough and socially isolated, childhood, "we became convinced that skill, courage and determination could triumph over almost anything and we tried to pass this on in our books." Thousands of young readers have relished her stories embodying this message, sometimes riders themselves, but also those who simply enjoyed stirring tales showing how plucky children, mostly girls, can still manage to overcome everyday adversity in a vanishing rural world so different from their own.

Josephine Mary Wedderburn Pullein-Thompson, author: born London 13 April 1924; MBE 1984; died London 19 June 2014.