A Theory of Fun for Game Design is a book written and illustrated by Raph Koster. It is based upon a presentation Koster gave at the Austin Game Conference in 2003, and the book reflects its origins by displaying text on one page and a cartoon/graphic from the talk on the other page.
Raph Koster's thesis is that games are all essentially "edutainment", teaching us the skills we might need in real life in a safe, low-stakes environment. A good game, according to him, is "one that teaches everything it has to offer before the player stops playing."(p. 46)