Sharon Wagner

American author. Born in 1936. Prolific writer of young adult novels, mainly  mysteries, and also of adult romances and gothic romances. Some of her young adult books feature horses, including  the well-loved Gypsy series which is a traditional series with a mystery element.  Sharon Wagner additionally writes as Blythe Stephens, although I don't think she wrote any equine themed novels under that name.

Gypsy Series:
Traditional/mystery pony series set on a dude ranch and featuring a Morgan horse filly.

1) GYPSY FROM NOWHERE
(WHITMAN/WESTERN PUBLISHING [USA] 1972)
ILLUSTRATED BY LOUIS CARY
Reprinted a couple of times in paperback.
SUMMARY: Wendy used to love horses and riding but after a horrific accident has become afraid. When her father has to go overseas to work she stays with her Aunt and Uncle on their ranch in Montana. Can they and the little lost Morgan filly Gypsy who appears from nowhere, encourage Wendy to ride again?

2) GYPSY AND NIMBLEFOOT
(WHITMAN/WESTERN PUBLISHING [USA] 1975)
ILLUSTRATED BY LOUIS CARY
Reprinted in paperback.
SUMMARY: After an accident, the Webster's pinto horse Nimblefoot becomes nervous and potentially dangerous. Wendy, who also lost her nerve after an accident, feels an affinity for the horse and despite her own fears hopes that she can help the pinto. But if she can't, Mr. Webster plans to have the horse destroyed. When Jimmy, the young boy who was riding Nimblefoot on the day of the accident arrives on the scene, Wendy hopes she can piece together what happened and find the reason for the horse's fear, but Jimmy is hostile and seems to be concealing something. Wendy and Gypsy must get to the bottom of the mystery in order to save Nimblefoot, And matters are further complicated by mysterious goings-on at a supposedly haunted nearyby gulch….

3) GYPSY AND THE MOONSTONE STALLION
(WHITMAN/WESTERN PUBLISHING [USA] 1980?)
Reprinted in paperback.
SUMMARY: Wendy and Gypsy find a mysterious wild stallion.

Collectors info:
All were reprinted in paperback by Golden Books, an imprint of the original publisher. Easy to find in the USA and cheap, may be a little harder elsewhere, although Amazon UK usually has some copies.

Other Pony Books:

PRAIRIE WIND
aka PRAIRIE LADY (Scholastic paperback edition)
(MEREDITH PRESS [USA] 1968)
ILLUSTRATED BY RITA WARNER
Reprinted in paperback by Scholastic in 1975 when it was re-titled.
SUMMARY: Molly finds an escaped horse which has got loose and is trapped by it's rope and starving. She is determined to save the horse's life.

THE DUDE RANCH MYSTERY
(McKAY [USA] 1969)
ILLUSTRATED BY TOM GODDARD
SUMMARY: Some horse content in this mystery set against a backdrop of a dude ranch in Montana.

Collectors info:
Easy to find in the USA, may be a little harder elsewhere, although Amazon UK usually has some copies.