Eileen Meyler

British author. Ms. Meyler wrote a number of children's books, including the long running 'Adventure' series about the Elwood Children. One of the series was a pony story. I don't think any of the others had any horse content in however.

Horse & Pony Books:

ADVENTURE ON PONIES
(EPWORTH 1959)
ILLUSTRATED BY RUTH SCALES
Reprinted in hardback in 1968.
SUMMARY: Set in Dorset. Part of the 'Elwood Children Adventure' series, but can be read as a stand-alone pony story. 

Collectors Info:
Although not commonly seen it is reasonably easy to find on the internet and usually low cost.