"Equine Fitness: A Conditioning PRogram of Exercises and Routines for Your Horse" by Jec Aristotle Ballou. So now my once tubby, hay belly awfulness, un-fit mare is now muscling out! When we lunge on hills I can see her core muscles engaging, even under her winter fluffiness!
I very strongly recommend this book for people of all disciplines. It breaks down the body parts of the horse and explains how the anatomy works, and then gives exercises, stretches, and massages to supple and condition your horse physically, mentally, and especially cardiovascular health.
The book lists not only individual exercises, but it also has routines for different health styles. Example: it has a gradual routine for building up the convalescent or injured horse. Or the senior horse. It is SUPER helpful and everyone should at least look though it to get some good ideas!