There has been nothing written for the World of Greyhawk that can be considered 'canon' since WotC/TSR stopped officially supporting the campaign. Heck, even some of the material released officially by TSR during the 2nd Edition AD&D days is not considered 'canon' by the die hard Greyhawk fans.
As a matter of fact, 'canon' is a dangerous word to use when talking about the World of Greyhawk. Just post a question regarding what is and what isn't canon over at the WotC Greyhawk messageboards or the forums over at Dragonsfoot.org and you're sure to start a huge argument between the different camps of Greyhawk fans.