Rel said:I hate to say this, but: The World Builder's Guidebook by Gary Gygax
Psion said:Important note: World Builder is the book by Gygax.
World Builder's Guidebook is the excellent and to date unmatched world building resource written by Rich Baker for 2e that I still use to this day.
Buttercup said:If you mean, which book have I not used because I can't find a game...