So now I've got an understanding on basic form and I'm happy to move onto learning to paint texture and materials like metal, wood, leather any texture but all textures. I'd rather learn how to paint or draw these texture's than to rely on photo texture's.

I get it's all about how light interact's with the surface's of the object but how to I learn to paint these various textures? with and understanding on how to replicate these onto work and studies.
Now with observational painting I'll do the studies but if I try it reproduce it on another object or study then it doesn't work, meaning that I don't properly understand why the object act's like it does.

Basically what I'm asking is are there decent book's or videos's on how to paint these textures and understand the Why and how the materials act that way?
Also I'm interested to hear how other learn't to paint.

Note this is for 2D art not 3D.

Any help is very much appreciated, Thank-you