The Allen Illustrated Guide to Bits & Bitting by Hilary Vernon

Invaluable guide for the modern rider, covering all types of bits, their correct fitting and action. This comprehensive reference book includes:

Pressure points explained simply 
Assessing the structure of the horse's head and mouth
Use of external and internals aids
Bit Accessories
Bitting materials
Measuring
How to buy a bit and what to look for
How different bits work
How to fit different types of bits
A pictorial guide to bits
Combination hackamorebridles