Reprinted under different titles: Poisonous Relations and A Taste for Murder
The Aston Park Guest House and stables does not mention, when one books, that murder is included. A guest is found dead one morning, and all there are suspects: the owner Sir Charles d’Estray, his wife and her daughter, and the guests.
For far more information on Joanna Cannan, see her page on my website here. Joanna Cannan wrote several adult books as well as her children’s literature. She wrote several murder mysteries, of which one, Murder Included, has an equine background.
Finding the book: not I think still in print, but reasonably easy to find.