Mary Adrian

American author of children's novels. Born in 1908. She has written over thirty books for children, both fiction and non-fiction. Animals and nature appear in a number of her books, especially in her non-fiction, Horses feature in one of her novels. This is part of a series of mysteries centred around various themes. 

Horse & Pony Books:

INDIAN HORSE MYSTERY
(HASTINGS HOUSE [USA] 1966)
ILLUSTRATED BY LLOYD COE
SUMMARY: Indian Jim investigates the mystery of missing cattle, whilst trying to tame a wild mare.

Collector's Info:
Reasonably easy to find in the USA. Much harder elsewhere and may be expensive.