Harriet Castor, who also writes as H M Castor, is now a writer of historical fiction. She studied history at Cambridge, taught English in Prague, and worked for the Royal Ballet as a dance notator. She was a phenomenally early starter as a writer: her first book was accepted by Penguin when she was 12. Since then, she’s written over 40 books, including a series about ballet, and a good mix of historical non fiction. Her one pony book is a contribution to the lengthy Sleeopver Club series, written by a number of authors.
Finding the books: easy to find: as Pony Passion the book is still in print.