Hilltop Horses was published by CSSM (Christian Special Services Mission), so I assume that the book has a Christian slant. As far  as I can tell, the author wrote just the one book under that name. As yet, I have not been able to dig up any biographical information about the author.

Finding the book
Easy to find, and not generally expensive.

Bibliography: pony books only
Hilltop Horses – a Tale for Children
CSSM, London, 1953, 63 pp.