Black Horses for the King

The publishers describes this as McCaffrey's "first historical novel for young readers", and it is probably not a genre book, in that there appears to be nothing actually fantastic about the story.
"This novel was developed from the author's story "Black Horses for a King," which first appeared in the anthology Camelot, edited by Jane Yolen and published by Philomel in 1995." In her introduction, McCaffrey credits Yolen with "all these 'minor' changes that became major improvements in my yarn."

Month from Locus1 which has a page count of 221 which includes unpaginated afterword. OCLC has a page count of 217.