I understand the original poster's desire for books. It is (perhaps) better to learn by seeing/doing? But if not available or even if it is, it's nice to lounge on the couch and learn what you're going to do next.
Book recommendations are dependent on what you're trying to do. If you're a rank beginner, I kind of like the Storey's Guides. They're not aimed at miniatures, but you can learn a lot that I see people inquiring on these forums over and over, regarding general horse care, barns, etc. The book I gained the most from is by Cherry Hill--101 Ground Training Exercises, which is not aimed at mini owners.
I'm going to pore over Annasdreamer's list to check for books I may have missed.... But there's one title I have to disagree with: Miniature Horses - A Veterinary Guide for Owners and Breeders By: Rebecca L. Frankeny, VMD
Perhaps I had inflated expectations but I was hoping for a veterinary reference. This book was not it. Obviously if there's serious issues, you take the animal to a vet. I don't know who the target audience was for this book. It was too simplistic for other vets, but had little to offer horse owners (in my opinion.) The best chapter in the book, although vague, was the chapter on Nutrition... somebody else-some dude wrote that chapter. Huge disappointment!