Horse & Pony Books:
(1st edition WORLD'S WORK 1970)
ILLUSTRATED BY ELSIE WRIGLEY
SUMMARY: The story of a strain of silver blue coloured wild ponies set on Dartmoor
I don't think Silver Blue was ever reprinted and is therefore fairly rare, although there are usually one or two copies around, in the UK at least. Price can vary a lot. The author's other animal stories are however much easier to find.