Fiona Citroen wrote this book when she was 24. As far as I know, she wrote nothing else under this name. The book is one of the last pony books to be published as a hardback with dustjacket: it became progressively the norm in the 1970s for books, even for sure sellers like the Pullein-Thompsons, to be issued first as paperbacks.
Finding the book: easy to find and usually reasonably priced.
Applegate - The Story of a Girl and Her Ponies
Young World Productions Ltd, London, 1972. Illus Michael Whittlesea, cover Michael
Jackson, 156 pp.
Madge Summers inherits Applegate, a riding stable. She has wonderful dreams about
the place before she gets there, only to find out it is extremely run down. So,
she sets about sorting it out.