Avril Knott wrote just the one pony book I have been able to find. She wrote it at the age of 15, and it is not particularly easy to find. Because of this, pricing can be erratic.
A Pony of Gold
Arthur H Stockwell, Ilfracombe, 1948, 176 pp.
“...the story of Fairest, a beautiful little golden-chestnut pony-mare, and Sandra,
her devoted fifteen year old mistress. The story covers Fairest’s training, hunting,
gymkhanas and a point to point.