As far as books are concerned, I highly recommend these 2 as must haves:
As far as everything else, go to as many trainers, barns, shows, vets, breeders, WHATEVER that you can!! PICK THEIR BRAINS!!
Watch them, AND listen!!! Most importantly, remember that there is more than one way to get something accomplished. Learn to recognize the methods that are common with a lot of people that get results, those are going to be the methods that are as close to a "right answer" as you will get. Be willing to try anything as long as you are patient, and learn to recognize AND ACCEPT when you are in over your head.
Make the key rules more of a natural reaction than breathing. Each of us have our own set of key rules, although they are probably all very similar.
NEVER EVER think you know even close to everything, the minute you think you know a lot is the minute you find out you know absolutely NOTHING! Even if there is a breed you don't care for, or a style of riding you don't like, LEARN about it, get a solid understanding of it's fundamentals, learn to at least appreciate what it has to offer. (example: I REALLY do not care for Apps / POA's, I just don't like them. BUT, I'm in a position right now that a POA would probably work out the best. And I accept that).
Learn to keep your mouth shut, lol. Seriously, it sounds crazy but the horseman that are the most admired are the ones that say the least. They say what needs to be said, nothing more, nothing less.
Don't be ashamed to admit when you don't know something, or you don't understand what is being asked of you. It could mean the difference between you going home in one piece or not.