I learned to dye wool while taking class with Dick LaBarge, a very well known rug hooker and teacher. Dick is retired now from teaching, but I'm told he still hooks with some of his former students.
Anyway, Dick was a master at dyeing. I always felt like I was entering a secret laboratory when he got the dye pots out. We had lots of fun experimenting with colors and techniques.
From that experience, I developed three dye booklets of my own. These booklets used different methods -- spot dyeing, transitional dyeing, graduated dyeing, abrashing, and overdye. My first dye book -- The Dye is Cast -- is only available as a .pdf, but my later dye books -- The Dye is Cast Again and Three Times a Charm are still available in printed format.
The next photo shows most of my dye formula booklets that I reference when dyeing. My favorites are the three Prisms booklets. I also use Vermont Folk Rugs book of formulas on a regular basis. The Prisms books use only ProChem dyes, while the Vermont Folk Rugs book uses Cushing dyes.

I also have compiled in a notebook all my favorite formulas. I have my own formulas, and other favorites from the books seen in the photo. In addition, I have a lined notebook where I write down formulas I want to try, mostly ones I have developed on my own. I try to keep up with organizing these formulas, but sometimes I get behind and then have to search out that scrap of paper where I wrote down a new formula. That is not fun!
What dye books do you use?