Marjorie Procter wrote two pony books set on Dartmoor, both of which feature the Sheppard family. Both titles were illustrated by Anne Bullen. As well as her pony books, Marjorie Procter wrote several religious works for younger children, re-telling bible stories.
Sheppard family books
The Pony Trackers
The Treasure Riders
Finding the books
Neither book is particularly hard to find, or expensive.