After last year’s excellent Rules of play (↵Salen & Zimmerman 2004) now everybody recommends:
 

KOSTER, RAPH. 2005. A theory of fun for game design. Scottsdale, Arizona: Paraglyph Press.
 

For background information see the according ↑entry at game matters with extensive comments, and ↑Conversation with Raph Koster by Celia Pearce. And if we’re talking about ‘definitive’ books on computergames, here’s a hint: Chris Crawford’s classic ↑The art of computer game design is online already since 1997. Just to round it up, the ludologist ↑points to ↑The evolution of gaming: computers, consoles, and arcade, another take on game history. And then, academics concerned with games, get ↑always_black and its repository of non-fiction articles, the ↑black_box on your radar, just as Rex ↑has proposed—tnx for that. Not that I am an evolutionist, stating that graphic novels are the evolutionary predecessor to computergames, but of course there are connections between those ‘genres’. Hence another classic:
 

MCCLOUD, SCOTT. 1993. Understanding comics: the invisible art. Northampton, MA: Tundra Publications.