The Hon Mary Colville (1922-1981) was the daughter of the Rt Hon John Colville MP (1894-1954), Baron Clydesmuir, who stood as a Scottish Unionist. Mary was part of a tranche of young authors who wrote pony books. Hers is one of the longer examples of the genre, though it follows the usual autobiography-of-a-pony plot. Plain Jane is the story of Blaese, a Shetland pony, and was illustrated by the author, who apparently had never had a drawing lesson but produced some pretty credible artwork nevertheless. The episodes described in the book which involve ponies were true.
Mary Colville married Captain North Edward Frederick Dalrymple-Hamilton of Bargany, and under her married name of Mary Dalrymple Hamilton, wrote As Lovely As A Tree. A Pictorial Album Illustrating The Many Common Trees In The Old Scottish Garden Of Bargany, Ayrshire.
Finding the book: reasonably easy to find though pricing can be erratic.