Seriously, if you are familiar with Greyhawk, and intend to use Oerth as your campaign setting, this can be an invaluable book for you. The detail of Iuz, his lands, clergy, motivations, enemies, etc...is spot on. As you well know, the three greatest evils to face the Flanaess are Iuz, the Scarlet Brotherhood (representing Tharizdun), and Vecna (at least that is how i perceive it), with a nod the the Elder Elemental Cult as well. This handles Iuz's part efficiently and in great detail. So many adventure ideas can be had simply by reading this one accessory, i consider it to be an essential piece of your D&D library, as it pertains to Greyhawk.
It can be difficult and/or expensive to acquire these days, but a worthwhile investment if you can keep the cost under $50. Just a further price tip...unless you are planning to buy and sell, get a game used copy. You will save $50 easily, and still get exactly what you are expecting.
If you have any other questions, feel free to write me. Otherwise....
Enjoy!