The World of Greyhawk is my absolute favorite setting. I love the 'Realms (especially the 1E Old Grey Box and the 3E FRCS) but Greyhawk is just my personal favorite thing, with all it's weirdness (in the original sense of Lovecraft/Clark Ashton Smith style Weird Fiction) oldschool 1930s pulp feel, and its mix of Late Dark Ages/Early Medieval Earth and Jack Vance Dying Earth. While the 'Realms are at a level equivalent to Earth circa 1400-1600 (Late Medieval to Renaissance) Greyhawk is at more of a 1000-1300 level. And I just love that. I loved the fact that you could walk into a major settlement, like the Free City of Greyhawk, pass by huddled beggars pleading for food (who were just as likely thieves ready to cut your purse if you stopped to help) and then continue on to the town square where there could be a subdued dragon up for auction! And I
absolutely loved the fact that it was all a mish-mash of Knights and Wizards and monsters and magic and psychic powers and martial artist Monks.
I love Alignment Languages!
It really did feel like an old 1930 weird pulp story, with everything thrown in just because it was cool and fun, with a touch of darkness, and the "heroes" were more likely to be glory-seeking, tomb-robbing roughnecks out for gold and fame than they were to be high-minded, world-saving pinnacles of righteousness.
I love the way the original maps didn't have any hard, definite borders and instead just had the names of realms in large letters at the center of the area they controlled, and the areas between just sort of drifted off and individual towns and cities mattered more than kingdoms and large realms. Just like in real life medieval Europe. It really fit the setting with the rules, how a 9th level Fighter could just go out into one of those "in-between" areas, clear an area of monsters, build a castle, and now he was Lord So-and-So and could collect taxes from anyone who came to settle in his newly cleared land. (I really miss that aspect from the game, by the way, and just wish there was more of that kind of thing in the 5E rules, which I mostly love to pieces.)
The books and boxed sets that I consider to be the best sources for Oerth are:
The 1983
World of Greyhawk Boxed Set, for Advanced Dungeons & Dragons 1st Edition
The
From the Ashes boxed set, for Advanced Dungeons & Dragons 2nd Edition
The Living Greyhawk Gazetteer, for Dungeons & Dragons 3rd Edition
I personally use these three sets as my primary sources of information for Greyhawk. Also, the original 1980
World of Greyhawk Folio is great if you can get a hold of it, although I consider the 1983 Boxed Set to be superior.
I was first introduced to the World of Greyhawk setting with From the Ashes, which I found on discount in the bookstore I worked at during my freshman year of college, and I immediately fell in love with the setting. About a year later I was introduced to the 1983 boxed set by my friend who became our primary Dungeon Master, who preferred running his games using the 1st Edition rules instead of the current (at the time) 2nd Edition, and he lent me his copy of the '83 boxed set to read to my heart's content because he knew how much I loved Greyhawk (he also outright gave me his copy of Forgotten Temple of Tharizdun because he knew how much I loved Greyhawk and how much I loved HP Lovecraft.)
You should be able to buy PDF copies of all of these books to use for yourself. As for myself, two months ago a very good friend of mine, one of the guys I play D&D with, was able to buy both the 1980 and 1983 Greyhawk sets, in very, very wonderful crisp, clean condition, as part of a large lot of old D&D books (including two copies of the 1983 set!) from an older gentleman who was selling off his collection, and since my friend knew how much I love Greyhawk he asked me how much I thought they were worth, and if I would be maybe be interested in just buying them from him before he put them up for sale, if I didn't have them already. I showed him how much they were going for on eBay currently (in much, MUCH worse condition!), let him know that I had desperately been wanting to buy the 1983 boxed set for years but just hadn't had the money, and told him that I could pay him 40$ for the 1983 set now with more to come later.
Instead, he made me a counter-offer of $20 each for both the 1980 and 1983 sets, and said he wouldn't accept a cent more than that from me! He told me that he would much rather sell them to me than to some random guy online because he knew that I would really love them, and he wasn't concerned about the money (he knows how many medical problems I have and that I live on disability income and just don't have very much money, and also knows how much I love Greyhawk.) So no I am now the proud owner of physical copies of all the major World of Greyhawk campaign books and boxed sets, all in excellent condition, including the original, beautiful Flanaess maps by DARLENE. The only thing I'm missing now is the box for the 1983 Boxed Set, so I'm keeping my eyes out for one that I could pick up.
But that's enough rambling for now... sorry, I tend to get carried away when I start talking about Greyhawk.
(My friend is a really, really awesome guy with a huge, kind heart.)