Which fantasy world has the most written about it by its creator(s)? This came to my mind when i was reading the review thread of Forge of Darkness. Erikson has written 11 novels, each between 700-1200 pages, plus 4 novellas, add to that Ian C. Esslemont's 4 novels, and more on the way from both. Maybe even this year. That's quite an extensive backstory for one world, especially to have been all published within the last 15 years. I'd be hard pressed to figure out how much has been written about Michael Moorcock's multiverse, but considering that you can include almost all of his fictional works in one world, and he's been at it for 50 years, I bet he's somewhere near the top. I specify creators to avoid inclusion of the extensive catalogs of Cthulhu mythos and Hyborian mythos that could be endless.