Frances Murray wrote three pony books I am aware of.
Finding the books
Ponies on the Heather is very easy to find, and cheap. Parachutes is slightly less easy to find, and reasonably priced. White Hope is findable, but can be expensive.
I’m a freelance writer, editor and proof reader. I’ve written Heroines on Horseback (2013), a gallop through the pony book in children’s fiction, and have written the world’s largest website on equine literature.