National Velvet by Enid Bagnold
One classic I haven’t read is National Velvet. You can tell because I’ve painted a show jumper not a steeplechaser. And Pi was a Piebald (paint) not a chestnut. But they didn’t get that right in the movie either, so I feel somewhat vindicated.
There was a horse called "Moiffa" who is sometimes referred to as the horse in the story Mi tells to Donald, that was shipwrecked, rescued and won the Grand National in 1904. But that's a bit of a myth.
The character of Velvet Brown was based off the author's horse-crazy daughter.