If "Greyhawk Castle" is what I think it is (i.e. WG7 Castle Greyhawk), definitely do not get it. Depending on your investment in the setting, it is either a comedy adventure that is deeply unfunny, or a deliberate insult aimed at TSR customers and/or former TSR directors.
I've never seen a copy of Greyhawk Wars, but it is a boardgame so probably not what you are looking for.
From the Ashes burns down the setting of World of Greyhawk 1983, creating the setting of (Living Greyhawk) Gazetteer. Never seen that one either (too expensive on ebay for my taste) but I find it quite intriguing.
Expedition to the Ruins of Greyhawk is published by WotC but is basically a Paizo love-letter to the city and castle of Greyhawk. I like it a lot, and there is tons of lore scattered about it, but it is primarily an adventure not a sourcebook. (Also, it's an incomplete adventure, as the ruins of Castle Greyhawk are way too big to put in one book; it needs a bit of work from the DM in order to run it.)
