Canonicity (for Quest purposes)
Tier 1: The Quest itself is primary canon.
Tier 2: WoQM applies unless it violates Quest canon (which I assume it has or will at some point).
Tier 3: Army Books (6th+), WHFRPG 2e - reasonably safe to assume that the fluff in these is canon unless the Quest or WoQM says otherwise. Game mechanics should not be taken as canon.
Tier 4: Black Library, White Dwarf articles - canonish, but the QM may not be familiar with them and the details are likely to end up varying if they are used.
Tier 5: Licensed video games, Warhammer Armies Project, WHFRPG 3e & 4e - mostly only used for things that aren't otherwise covered in higher tiers, and by default are not canon.
Tier 6: Army Books (pre-6th), WHFRPG (1e) - the Dwarf Priests Know Necromancy Zone. May be looted for ideas from time to time but is usually completely incompatible.