If there's one book that a DM has to have if their going to run Greyhawk in the minds of many players of AD&D or Castles & Crusades. It has to be James Ward's Greyhawk Aventures! Why?! Because of the fact that Greyhawk Adventures expands Greyhawk of course! Expands upon the dieties, the monsters, Hall of Heroes places, persons, & things of Greyhawk, huge nimber of magical spells, new Greyhawk magic items, the geography of Oerth, adventures on Greyhawk, zero level PC options & available customization, and a Greyhawk spell list.
So while all of this is packed into here running Greyhawk with Castles & Crusades is going to be a bit of a challenge. Greyhawk Adventures helps a ton because it gives the dungeon master the first real expansion on Greyhawk since the classic box set.