Horse & Pony Books:
SPURS FOR ANTONIA
(OXFORD UNIVERSITY PRESS [USA?] 1943)
ILLUSTRATED BY DECIE MERWIN
Reprinted in hardback by original publishers a number of times.
SUMMARY: Antonia moves to California to live with her father on his ranch when her Aunt who had been looking after her dies. She soon adjusts to ranch life and learning to ride but feels she must prove to her father she is as good as a boy.
STAR IN THE WILLOWS
(OXFORD UNIVERSITY PRESS [USA?] 1946)
ILLUSTRATED BY GERTRUDE HOWE
Reprinted in hardback by original publishers
SUMMARY: Mexican girl Nita and Californian Lou are united in their love of horses. When Lou's horse gives birth to a palomino foal they name Star, Nita must protect the little colt from a mountain lion!
Reasonably easy to find in the USA. Star in the Willows also seems to be available in the UK, although Spurs for Antonia not so. Both hard to find elsewhere.