The Art of Videogames A must-read for people who want to know where videogames stand in the philosophy of art.
Reality Is Broken: Why Games Make Us Better and How They Can Change the World Will games save us from ourselves? I was skeptical, but Jane McGonigal has made a persuasive case. There's something to her argument.
Flow: The Psychology of Optimal Experience (Harper Perennial Modern Classics) It's hard to overstate how important Csikszentmihalyi's work is for an understanding of game design.