FLASH OF PHANTOM CANYON
(CRITERION [USA] 1963)
Scholastic paperback edition also published in 1963 and later reprinted.
There was also a German printing.
SUMMARY: As he cares for a wild horse, Gabriel, a troubled young boy who is half white, half Indian, learns to accept his real identity.
Easy to find in the USA. Harder to find elsewhere, although there may be one or two copies to be found on the internet.