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Wyatt Blassingame (1909 - 1985) wrote over 600 short stories. His full length books were generally non-fiction; strictly speaking this horse book is that too, concentrating as it does on famous horses like Buccephalus, Rienzi and Justin Morgan.
Finding the book: reasonably easy to find.
Links and sources
Wikipedia on Wyatt Blassingame
Bibliography - horse books only
His Kingdom for a Horse
Watts, 1957, 179 pp, illus Sergei Korolkoff
A collection of stories about famous horses, such as Buccephalus, Rienzi and Justin Morgan.