Hey there! Late to the party, but wanted to chime in. Greyhawk's my home campaign world (shared campaign with a group which shifts DMing duties) and I've read most of what's out there (including fan sources, but not including Living Greyhawk modules of which there were 1000s). Adding to what what others have recommended:
Light Intro, overview: "The Adventure Begins" book, 2nd edition era.
Great intro for players, or DMs who want a quick overview. Modern-ish sensibility of setting-book organization and topics. It was written at a point in the timeline that had some important, world-shaping events which mattered for later products (namely the 3.5-era adventures, more about those later). It's not exhaustive, and a bit rinky-dink to use on its own.
Deeper reference, overview: Living Living Greyhawk Gazeteer, 3.0 edition
For modern D&D players who never lived through the 2e or 1e eras, I recommend this book before the World of Greyhawk setting as it has a really clear, if dry approach. It's definitely like an encyclopedia, with Paizo editor Erik Mona's signature depth, and tons of hooks/potential.
It also covers even more events that were added after the WoG boxed set than The Adventure Begins does, This includes the Greyhawk Wars, which reshaped the map into what's in the 2024 DMG now. Oldschoolers pooh-pooh those advances of the plot, but IMO they were well done. The former juggernaut of the continent, the Great Kingdom shattered; giants conquered large portions of the map; secret societies struck from the shadows and subverted several nations, but were heroically resisted in others
Adventures, examples of oldschool/classics: The Classic A1-4, then G, D, and Q series
The Classic G, D, and Q series of adventures are great flavor, and make a cool campaign arc when played in sequence. Doing so will definitely give your players a view of many of the setting's classic bits. They are not however modern adventures; the format is a bit spare, and some railroading etc. can happen here. You'll want to adapt a lot. Fortunately a lot of forums had suggestions for improving these adventures. You may have to use the Wayback Machine to access some of those forums, but that's modern life right?
Adventures, really playable/convertable material: 3.0-3.5-edition Dungeon adventure and Dragon Magazine articles.
These are the crown jewels for a 5th/6th player group wanting to use Greyhawk material with good plot and relative ease of conversion, IMO. The Savage Tide campaing arc from 3.5 was particularly good, but the Whispering Cairn (first adventure of the Age of Worms Campaign) has amazing tie-ins to classic Greyhawk lore, as do the other four or so adventures of the beginning of that arc (which fell of a bit at the end IMO). There's also the Shackled City, a campaign which feels oddly like a China Mieville book. There are tons of other good stand-alone adventures in Dungeon over this period.
There were also some good short-arcs or cobbled-together sets of missions, notably a few set in the frozen lands of Blackmoor to the north (those are really cool adventures imo) and the Seeds of Sehan arc set in the Yatil Mountains (cool Tibetan-style mountain stuff in there plus interesting urban/investigation adventuring).
Finally, the oldschool trump-card in 3.5-era Dungeon was the Maure Castle set of adventures, written by Rob Kuntz . Rob was the third Dungeonmaastter ever, being Gygax's neighbor kid who became his co-DM. He helped form the idea of how "dungeon" and world-plot came together, and how people could play with the rules and story together at the table. The arc starts with an adventure which is a revamp of one of the most classic early D&D adventures, WG5 "Mordekainen's Fantastic Adventure..." but your players come through 30 years after that great mage looted the place, and deal with the aftermath of Mordy's early-adventuring-career approach to hack and slash looting.

He then added a couple more levels to the dungeon in later issues. These adventures are classic dungeoncrawls, but with a deep understanding of what the game can be underneath them, rooted in a literal lifetime of DMing and writing. They're higher-level (12th through 20th), so best to save for when your players know the world of Greyhawk a bit.
Meanwhile Dragon had in-depth articles on major Greyhawk deities where they fit the 3.5 core setting, and some great focus articles about setting areas like cities (Greyhawk, Hardby, Exag, etc.) and regions (Bright Desert, the Horned Society lands, Blackmoor, etc.)
Mona and Co were really, really good at this point. If you read nothing else, read through the Whispering Cairn.
Fun read with really evocative background info: Living Greyhawk Journal
To support the worldwide Living Greyhawk Campaign, Paizo published this series in Dragon Magazine. It had really great articles on things like the different ways paladins or other classes exist in different Greyhawk cultures (with specialized classes or efats for each); major power groups like the Silent Ones of Kepland who seek to prevent dangerous magic items getting loose, and a series of unique creature profiles written from the perspective of an academic demon serving one of the undead lords of the Great Kingdom (really, it's brilliant writing).
The material is eclectic, but there's good depth on certain things which were major Living Greyhawk adventure arc fodder, like the Bright Lands (which was not well described in earlier material either).
I think it was also published stand-alone for a while (iirc?), but there is a collected volumes 1-25 file out there on the internet, I believe legally and for free, that I highly recommend.
Once your group knows the setting well: Expedition to the Ruins of Greyhawk
A lot letter to the setting, well-written and lovely illustrations. I would recommend you not play it though until you group has the background info to really appreciate the lore and characters woven through it, even more than the Maure Castle adventures.
Fanzines: Oerth Journal and Greyhawk Grimiore
Oerth Journal has great in-depth writing on regions, cities, etc. Many of the published canon folks were in there, and the editors tendedd to be pretty clear about what material splits off into "Infinity Oerths" territory of being explicitly alternate from canon. The stuff that styed closer to the bone was frequently good enough that I use it as much as the Livign Greyhawk Gazeteer when I want greater detail on cities, etc.
Online Support:
As others have pointed out, Canonfire is great. The boundary between fan and canon author really blurred there a lot, as various Paizo folks were in that community and many of the Greyhawk fan-writers there were published in Dungeon or Dragon sometimes.
The Sages of Greyhawk facebook group is decent for factual or reference questions, if you do the book of faces. There are various other Greyhawk groups like Greyhawk Adventures, but they tend to be fiefdoms a bit. Sages has the highest-quality posters for asking questions of, IMO.
Lastly, feel free to write me here if you have questions.
Hope that helps!