Exellent guide, just want to add few words.
Texture addressing and behavior is mostly driven by surface.txt
There you can find multiple texture entries for different effects (*.d,*.n,*sg and others, I think their responsibility like color, normalmap, glossmap, etc is defined by the number of the texture entry, so *.d is always texture0 and is a color texture, *.sg is texture1 and is a glossmap, *.n is texture2 and is a normalmap, texture3 is *.h or *.hx and defines where a player's skin color is applied to items).
Also there are very interesting flags under texture entries.
It seems they define behavior of alpha layer of *.d texture, for example SURFACE_FLAG_OPAQUE makes alpha work like glass, when SURFACE_FLAG_TENERGY is a t-energy inserts and SURFACE_FLAG_MASKED makes parts of surface disappear like net skirt in the tutorial. I guess there could be some other flags and combinations.
Next is shader which is applied to the surface and makes it glow like skin, plastic, metal, etc (full list of shaders is in the beginning of the file and they seem to be editable also).
So there are vast possibilities to modify items and surfaces look ingame by using surface.txt file.
The surfaces are adressed to each model in itemtype.txt.
And one more tip on editing surface.txt: if you mess something in it, the surfaces (and objects due to lack of them) won't appear ingame past messed part of the file. So there would be only surfaces which entries are before the error.
P.S. Well, maybe everybody already knows that, but possibly it would be useful for someone.