Rose-Mary Silvester wrote two children’s books for Christian publishing house Pickering and Inglis. The other, Ship in the Moonlight, is, as far as I can see, a straightforward adventure story. She contributed a short story, Thunder – Horse of the Storm, to Pony Magazine Annual 1967, which was illustrated by Joan Wanklyn.

I haven’t yet been able to find any biographical information about the author.

Finding the book
Not particularly difficult to find.

Sources and links
The British Library

Bibliography: pony books only
Cousins at the Manor School, Pickering & Inglis, London, 1969, 160 pp.