Warhammer's Realm of Chaos Slaves to Darkness (1988) and the 1990 follow-up The Lost and the Damned. God! Damn! These are awesome. I'm not that big into artwork, doesn't really do it for me in an RPG book---but the artwork in these books is heads and shoulders above anything I've ever seen in any RPG product---ever. And the content is doubly awesome. Massive work of the imagination, full of all kinds of useful stuff related to demons/daemons/chaos.
Zak S. mentioned it offhandedly in one of his Vornheim/Grindhouse podcasts, and I'm glad I caught the reference.
Get them. Get the older versions, before they got all Puritan with the art.
How come they don't make shit like this anymore? Seriously? The company who can put out stuff like this today WINS. Period.