Just so you understand, a visual approach is only ok to show somebody how to adjust a shader that has been spoon fed to them.

When I was learning Poser in my first couple years, I asked many people for help on a good leather shader. Leather is something everybody is intimately familiar with. We all know what it looks like. But no Poser "artist" was able to point me to any leather shaders except those that were part of products, that were drawn.  Nobody was able to make a convincing procedural leather.

Now, to me, seeing examples of how to draw leather is useless. I can't/won't draw, because I suck at drawing, and because after I'm done I have a drawing, basically one shader, not 1000 variations on it.

Slowly I developed an understanding of the math used to manipulate Perlin Noise, and was able to connect the dots and make a convincing procedural leather shader that I can instantly apply to all sorts of things, in infinite variations of tone, color variation and patterns, roughness, shine, etc. And I mean instantly.

Now if you want to know how this leather shader works, to the point that you could make one or something similar (for example, a real granite shader, which has a lot in common with leather), then you have to learn the math. I don't mean calculus. I mean basic algebra. Like how Bias works. If you can't look at a bump map and understand how to manipulate it with a couple nodes, then you're going to have to draw the bitmap over again, which is not shader work, but texture work.

You want a book on shaders, there will be math. You want a book on textures, there will be drawing.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)