Some friends and I did a lengthy study group on natural dyeing over the past 3 years. This book was an invaluable resource. Jenny Dean has dyed with everything. Her recipes are friendly and her instructions clear.
The first section of the book is a history of the use of natural dyes. Then she has a section on Techniques. This section is very useful. Read it thoroughly and keep it handly.
The vast remaining section of the book is organized by dye plant. She includes instructions for working with 66 different plant dyestuffs. (No cochineal, no mushrooms/lichens)
Each dyestuff has a description of the material, its cultivation and harvest and a description of the procedure used to extract the dye and put it on your fiber. There are color swatches along the edge of each page that will give you an idea of the types of colors you can get from that plant using different mordants and dips.
I enjoyed using this book very much - so much so that when the new edition came out, I made sure to get one. This is an indispensable resource for anyone interested in natural dyeing.
Here's a link to get one: