Horse & Pony Books:
PRETTY PENNY FARM
(WILLIAM MORROW [USA] 1987)
Reprinted in paperback by Troll.
Reprinted a number of times in paperback by Scholastic.
SUMMARY: Spending the summer on a New Hamsphire farm, fifteen-year-old Beth trains a beautiful chestnut horse for a local race and develops a friendship with an unpopular classmate.
Easy to find in the USA, especially the paperback edition. Quite a bit harder elsewhere, although can be bought from the USA fairly cheaply.