1) SUMMER OF THE STALLION
(MACMILLAN [USA] 1979)
Reprinted in paperback.
ILLUSTRATED BY GLORIA SINGER
SUMMARY: Janey has been looking forward to getting away from the problems of her everyday life and spending the summer vacation at her beloved Grandfather's ranch. But things don't go to plan when she falls in love with a troublemaking wild stallion whose spirit her Grandpa is determined to break. Can she save the stallion?
2) WINTER OF THE OWL
(MACMILLAN [USA] 1980)
Reprinted in paperback by Scholastic a couple of times.
SUMMARY: Janey wants to keep the son of the wild stallion, but her father and grandfather want to sell him.
Both easy to find in the USA. Harder elsewhere, though can usually be bought from America fairly cheaply.