As Ariosto pointed out, what do you do when the main campaign book actually points out that all these other sources are canon in the setting?
As I basically stated in the forked thread this one inspired...I say "Tough cookies!"
If info for an RPG that has generated published fiction isn't in the actual RPG books themselves, that non-gamebook info is relegated to "alternate universe" status.
OTOH, if it is an RPG that is based on a piece of fiction, I adopt that fiction as core info- assuming its (inexplicably) not already in the campaign sourcebook.