I have explored art and design for the last 30 years: creativity, fantasy design, jewelry-making, color theory, drawing, painting, model-making and all kinds of arts and crafts, both digital and traditional. I also have extensive experience writing ebooks and teaching online for since the beginning of the web, for almost 20 years. I'm sure you're going to love my ebooks!
I would definitely suggest starting with Eni's book 3D Tangle: Shading Fearlessly. And if you're interested in using gray markers for shading, buy her new Shading with Gray Markers. All of her books are excellent!
Shading Fearlessly will teach you a lot, even if you are already experienced.
Shading Fearlessly is fantastic, even when I am "lazy" about my shading, it still looks better than my shading before I bought the book!
...honestly have you gotten Eni's book? Shading fearless. She takes you through the steps so easy. Really worth every pennie. Once you do the trial samples you will be amazed at your progress. I promise...