Greyhawk, also known as the World of Greyhawk, is a fictional world designed as a campaign setting for the D&D fantasy RPG. The world itself started as a simple dungeon under a castle designed by Gary Gygax for the amusement of his children and friends, but it rapidly expanded to include not only a complex multi-layered dungeon environment, but also the nearby city of Greyhawk, and eventually an entire world.

Source: Portal:Novels - Greyhawk Wiki (greyhawkonline.com)

If you are interested in the suggested reading list located in the back of the 5e Players handbook, check out this reading challenge.  Dungeons & Dragons Player's Handbook Inspirational Reading | The StoryGraph

If you are interested in other campaign settings novels, check out these reading challenges: