Earlier in the year I stumbled across The Handbook of Natural Plant Dyes by Sasha Duerr.
I immediately ordered it and all I can say is it is just brilliant, after years spent piecing together bits of natural dyeing information from various sources and having hit and miss success with any project I tried, the book was like all my Christmases coming together.
One of the first projects I tried out was to dye a piece of organic cotton with blackberries. The instructions were straight forward and easy to follow. I definitely recommend this book for anyone with an interest in natural dyeing, an investment that you will spend years delving in and out of. The book is available to buy online here