The usual suspects by Enid Blyton, Arthur Ransome and Susan Coolidge, including the later Katy series. Rose Red was my absolute favourite character of children's literature ever: I had a bit of a girl crush on her!
Favourite ever: Penelope Farmer's Charlotte Sometimes. Haunting, beautiful story and I also liked the song by The Cure.
Loved 'Heidi' as a child, although as an adult I find it quite saccharine.
Horse stories: Jean Slaughter Doty's 'Can I get there by Candlelight,, Michael Hardcastle's 'The Saturday Horse' and Anne Digby's 'A Horse Called September'.
Also:
The earlier instalments of the Trebizon school series.
Alison Uttley's 'Little Grey Rabbit' books.
Nina Bawden, 'Carrie's War'
Jack London, 'The Call of the Wild'
Lewis Carroll's Alice books
Gene Kemp, 'The Turbulent Term of Tyke Tiler'