There are bits about the different kinds of bits (like http://www.amazon
.com/The-Ultimate-Book-Horse-Bits/dp/1628737379/ref=sr_1_1?ie=UTF8&qid=1384298304&sr=8-1&keywords=book+on+horse+bits ) , and there are books about how to (re) train the mouth/balance (like http://www.amazon.com/Twisted-Truths-Modern-Dressage-Alternative-ebook/dp/B00B0SA9CA/ref=sr_1_1?ie=UTF8&qid=1384298342&sr=8-1&keywords=philippe+karl ).
I have learn by training by masters for many years, and experimenting. Horses with problems must be restarted from the beginning. That means lungeing from a caveson, learning (with work in hand first) HOW to RESPOND to our ACTIONS. Learning how to be balanced/how to mobilize the jaw/how to teach the horse to seek fdo METHODICALLY. Almost always (in today's world) the snaffle is used for longitudinal flexion (which is not its intension). The snaffle and the curb have totally different uses. And too many things horses are happy with a bit when they get submission, rather than specific RESPONSES. That means the rider have CALCULATED tact and timing as well as TRAINING METHODOLOGY. This was true until the 80s, when submission and longitudinal flexion became the driving forces.
Although horses may toss there heads/etc because of previous poor training, it is up to the new rider to start with education and teach the horse how to properly respond. It is a methodical process from in hand (even w/o bits) to proper lateral flexion to gradual longitudinal flexion