Josephine Pullein-Thompson (1924 - 2014 wrote a few adult novels. The are all detective fiction, into which field she followed her mother, Joanna Cannan, who also wrote several detective novels as well as pony books. She also wrote one book under the name Josephine Mann. There is much more information about Josephine Pullein-Thompson, and her life and works, here.
Finding the books: tricky, and expensive, to find as first editions. As large print editions, they are much more common. The books are now being republished by Greyladies Press.