Dans,
I have a book called the Complete Guide to Natural Dyeing by Eva Lambert & Tracy Kendall. The book is good and has some lovely coloured pics but not everything I want to know is in there.
I bought my coloured extracts from this place
http://www.wildcolours.co.uk/ and have got quite a bit of information direct from their website. They have a book too which I have ordered but I think its out of stock at Amazon although they haven't come back and said so yet.
There are also quite a lot of uTube videos and mostly they have shown me not to be too hung up on specifics as they seem to put lots more wool in the dye bath than is recommended etc.
It seems the way to go is to follow a set recipe for the first couple of times (thats where I am at) and them experiment.
Have fun
Sally