B. Holland Heck

American author of children's novels. Also known as Bessie Holland Heck, although her books are listed under her abbreviated name in most book shops. Known for her authentic Oklahoma settings and was selected for the Oklahoma Writers Hall of Fame. Her stories were mainly of the ranching or adventure types and possibly aimed more at boys than girls. Two of the ranching stories feature horses. 

Horse & Pony Books:

CACTUS KEVIN
(WORLD [USA] 1965)
ILLUSTRATED BY LORENCE BJORKLUND
SUMMARY: A warm humourous story set in 1940s Oklahoma. Young Kevin longs for a horse like the palomino Jim when his family move to a ranch, but all does not go well for himů.

GOLDEN ARROW
(SCRIBNERS[USA] 1981)
ILLUSTRATED BY CHARLES ROBINSON
SUMMARY: Randy loves motorbikes and wants to be a professional motorcycle racer. In order to earn money to buy a bike he begins to work on his grandpa's farm, but after spending time with the horses, in particular the palomino mare Goldie, he decides that he likes horses better than motorbikes and wants to work on a ranch instead of being a biker (sensible chap!)

Collector's Info:
Fairly easy to find in the USA, harder elsewhere.