Judith Campbell (psued Marion Stapylton Pares) wrote on various equine subjects, including Your Own Pony Club (keeping a pony on a shoestring), police horses and showjumpers. She is probably best known for her works on the Royal Family. She wrote Anne, Portrait of a Princess with Princess Anne, and has also written on other members of the royal family (the Queen, and Prince Philip) as well as several books on the royal horses.
She wrote at least two pony stories: The Merrow Ponies and Four Ponies. Both books are about the Merrow sisters.
Finding the books
Neither is easy to find, and tend to be very expensive if you can track them down.
Links and sources
Many thanks to Lisa Catz for the photograph.