Carol Vaughan isn’t particularly well known now:  the first of her books about the Percheron, Matilda, was published in paperback but none of  her other books, as far as I know, were.  Books about heavy horses are not common, particularly Percherons.  Carol Vaughan’s books are amongst the most attractive of pony books visually:  I love Constance Marshall’s illustrations (and have a theory that she did some of the Armada Jill covers, but I’ve never been able to prove it), and the Michael Charlton one for King of the Castle is a delight.