Complete Kobold Guide to Game Design - Rules of Play says:
Wolfgang Baur, Monte Cook, Ed Greenwood, Mike Stackpole, and other industry veterans have the answers you asked for...and the advice you need! You'll learn how to create great new adventures, monsters and magic for your RPG campaign, and maybe even design your own game.
The Complete Kobold Guide to Game Design offers 240 pages of in-depth essays on what makes RPGs tick.