When I got serious about art, I started by redrawing Eddie Poferl stuff. Then I bought the SJ books, started drawing those, and redrew the Eddie Poferl stuff because I had 'cleaned up' the EP stuff the wrong way. IMO, SJ is still the best for learning shading and layout. Now I go out and find the stuff they referenced in the first place and redraw it for efficiency of line.
It may sound weird for a non-tattoo artist to go through this process, but when it came to learning how to draw and paint powerful images, there isn't a system that works as well this has for me.