The book by Alois Podhajsky, about his experiences with the Spanish Riding School, and horses. It his story during WWII in the movie The Miracle of the White Stallions by Disney.
Books by Mark Rashid, Considering the Horse is 1st one, there are maybe 5 or more? Once he gets into martial arts he kind of lost me...
Hold Your Horses, by Helen Crabtree, about her years as a Saddlebred trainer.
Last of the Saddle Tramps.
Any books by Ben K. Green, even cow books are great.
Saddleseat Equitation by Helen Crabtree.
Adventures of the 700 Pony, and it's sequel.
The Horses of Gentle Spirit.
Any of the Legends books by Western Horseman. And their book Horseman's Scrapbook.
Black Gold, King of the Wind, Black Beauty.
The 80 dollar champion (book about Snowman).
Various books on disciplines, breeding, breeds, farms, ranches, vet books.
If it is animal related. It is one of my favorites.
Horses make me a better person.