I'm pretty much going to second Cone of Negative Energy - except for one detail. I don't know how far we can nest things, but I think the distinction between Role Playing Game and RPG Supplement is very blurry _and_ also repeated in other areas, for example you might have a Board Game and a Board Game Extension or a Board Game Variant.
I think it'd be better to have a hierarchy or tags:
So, top level: Role Playing Game (this is the type of game).
Second level: Rule Book, Supplement, Adventure Module, Setting, etc.