Gordon Douglas Griffiths, as far as I know, wrote just the one pony book. It is set on Dartmoor.
Finding the book: it has now (2010) become, for no very good reason, quite extraordinarily expensive. Copies were easily available for much less until quite recently.
Silver Blue - a Story of the Ponies that Run Free on Dartmoor
Tadworth, World’s Work, 1970, Illus Elsie Wrigley