Sony Online Entertainment creative guru Raph Koster's book, "A Theory of Fun for Game Design", has just been published. It evolved from Raph's 2003 talk at the Austin Game Conference. The book addresses such essential design subjects as "what makes a game fun"; "whether games can be art"; and, above all, "why games matter."
Check out an excerpt from "A Theory of Fun for Game Design" at the gaming site, Gamasutra. (free membership required)