Mona Sandler wrote three books that I have been able to trace. Riding with Simon, with the wonderful cover illustration by Michael Lyne, is a non-fiction story about Simon, a grey Anglo-Arab whom Mona bought in a very poor state, and whom she eventually learned to ride. Her other two books are fictional, and are about the same two characters, who start a dealing stable.
Finding the books
Riding with Simon is reasonably easy to find. The two Horse Dealer books are harder to find, and appear to be getting more and more difficult.