Virginia Clark was a pseudonym used by Patsey Gray, about whom you can find more information here.
Finding the book:the book is not hugely common, and pricing is variable.
Links and sources
Terri A. Wear: Horse Stories, an Annotated Bibliography, Scarecrow Press, 1987
Bibliography - horse books only
The Mysterious Buckskin
Macmillan, New York, 1960, 164 pp
Ann and Ken take three dude riderss and their horses on a long trail ride. They have a buckskin horse, which Ann thinks looks just like her Frosty, stolen a couple of years earlier. Ken was actually accused of stealing the horse.