Mary de la Mahotière wrote just the one pony book: it is a sequel to The Newspaper Children, which, as far as I know, is not a pony book. The book is illustrated by Marcia Lane Foster, who was particularly notable for the way she designed her jackets so that the spine was an integral part of the whole.
Finding the book: generally easy to find and reasonably priced.
Round-up on Exmoor
Lutterworth Press, London, 1961, illus Marcia Lane Foster
- a story about the Newspaper Children. Roberta, Sammy and Bill are invited to join
a round-up
on Exmoor. There are more than 40 missing ponies to be found: even when they are
found,
more disappear later. They are determined to solved the mystery of what the crafty
bailiff
Fly Fletcher is doing with the ponies.