Theory of Fun

If there is one book on games design or games theory you must read it is “A Theory of Fun for games design” by Raph Koster.
I got the final one from Amazon last week (sorry) and read it in one sitting. Incredibly well-written, covering all sorts of topics from sociology to psychology as it applies to games. As I was reading, lightbulbs were going off in my head, and I could feel neurones making connections for ideas that should have been obvious to me.
The complete opposite of a dry textbook, this is fun and relevant and you owe it to yourself to read it!
Only you won't get it from Amazon at the moment… :S