This information is taken from "Exposure and Lighting for Digital Photographers Only" written by Michael Meadhra and Charlotte K. Lowrie
The Zone System was made famous by Ansel Adams and originally designed for B&W photography. It's an excellent tool to help photographers develop a better understanding of the interrelationship of exposure and tonal range, which in turn makes it easier to predictably produce properly exposed images.
The zones progress from absolute black to pure white, with each zone being twice as bright as the next darker step. Thus the zones are arranged in increments that correspond to one stop of exposure (one EV)
Zone 0: Pure black, no tone or texture whatsoever, absolute darkness, or its photographic equivalent.
Zone I: Near black, barely discernable tone, no texture - deepest shadows and silhouettes - dark end of the dynamic range.
Zone II: Textured black, very dark but showing a hint of textural detail - deep shadows on dark hair or cloth - dark end of texture range.
Zone III: Dark gray, shows distinct tone and textural detail - detailed shadows such as the dark bark on the shaded side of a tree
Zone IV: Medium-dark gray, an open dark gray with excellent textural detail - open shadows such as the shadowed side of a sunlt building or a strongly lit portrait, dark green foliage
Zone V: Middle gray, the tone of a standard 18% gray card - a luminous shadow such as the shadowed side of a low-contrast portrait, weathered wood, or dark stone - the standard target value for light meters
Zone VI: Medium-light gray, a bright midtone showing sharp detail - average Caucasian skin in good lighting, a concrete or stone building
Zone VII: Lightest gray, the brightest tone that still shows good textural detail - gray hair, light colored cloth
Zone VIII: Textured white, very light but showing a hint of textural detail - highlights on light skin, textured snow or white sand - light end of the texture range
Zone IX: Near white, barely discernable tone, no texture - highlights on white or light colored objects such as an egg shell or teacup - light end of the dynamic range
Zone X: Pure white, no tone or texture - specular highlights and light sources
This book is amazing. I can't compare it to any other exposure specific books. I just feel that this book explains the elements of lighting and leaves it to you to take what you read and use the information to learn to take better pictures. It doesn't tell you how to take the picture, it helps you understand what your doing and why things work.
There is more to using The Zone System, but it's way too much to write and there are pictures to help you understand how to use it.