Jessica Townend’s Nevermor series
At her age I devoured Elyne Mitchell’s Silver Brumby books - highly recommended for horsey girls.
Saddle Club books vary a lot in quality, but there are a lot of them.
KM Peyton’s Flambards series are classic horse stories.
Then there’s My Friend Flicka and following.
Meg Cabot’s The Princess Diaries.
Depending on her patience for slightly old-fashioned language, The Secret Garden and A Little Princess by Frances Hodgson Burnett might appeal. Ditto Anne of Green Gables.
Another classic is Elizabeth Goudge’s The Little White Horse.
There’s Misty of Chincoteague, National Velvet and Black Beauty.
Another classic series, not about horses, is Alan Garner’s Moonstone of Brisingamen. He is a wonder writer.
Susan Cooper’s Over Sea, Under Stone books are wonderful.
I should not forget Diana Wynne Jones. She writes for a variety of ages, but Howl’s Moving Castle should be bang on.