This book is published by Herm Sprenger and has illustrations explaining their technology and the research used to develop their bits.  X-rays of the horse's mouth, diagrams and photographs all make this an extremely informative read.  A well spent $10 before making an investment in your bits.

From the back cover:
"What is the best bit for my horse?" we are often asked.  There is no ready-made solution for this, but obviously the best bit is the one with which both horse and rider feel more comfortable.