What are some good horse starting books?

A good horse starting book is horses for dummies.It is available in many editions.
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This book was required reading for the colts in training class I had to take: "True Horsemanship Through Feel" by Bill Dorrance and Leslie Desmond. You could also try the book "There Are No Problem Horses, Only Problem Riders" by Mary Twelveponies. For working on balance and horsemanship, Sally Swift’s "Centered Riding" books 1 & 2 are rather good.
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You might want to narrow down what kind of book you’re searching for first. You say your gelding is broke and has "tons" of WP training. You also say you’d like to work with the gelding along with your trainer.
So, are you really looking for a horse starting book or a more advanced book? Are you going to be riding your horse WP or do you want to expand into a different discipline? If you’re paying a trainer, you want to make sure anything you do with the horse is in line with your trainer’s methods or you could wind up with a confused young horse.
If you are looking for a good, basic colt starting book, I found a great training guide recently. It’s a true, step-by-step training method for starting colts by an experienced, resistance-free trainer. I believe his method would work for starting pretty much any horse or pony.
References :
http://www.horsetrainingadvice.com