Mildred Mastin Pace (? - 1992) was born in St Louis, and moved to Kentucky at the age of 15. She worked her way through college by selling articles to newspapers, and carried on writing once through college. Her works were all (as far as I’m aware) non fiction, and included Clara Barton (1941), a biography, which won the Herald Tribune Spring Festival Prize for most outstanding children’s book for the older age group. Probably her most famous book is Old Bones, the Wonder Horse (1955), the story of Exterminator, a famous race horse nicknamed Old Bones because of his bony ugliness as a foal, but who proved a champion despite his looks.
Finding the book: very easy, though very good firsts with good dustjackets will be pricey. It’s available as an ebook.