Peter Mawhorter offers up a reading list on games:

For anyone curious about what I’ve been reading, here’s the list of what I’ve read to get an introduction to this area:

• “Why We Play Games: Four Keys to More Emotion Without Story” by Nicole Lazzaro.
• “GameFlow: A Model for Evalucating Player Enjoyment in Games” by Penelope Sweetser and Peta Wyeth.
• “An Experiment in Automatic Game Design” by Julian Togelius and Jürgen Schmidhuber.
• A Theory of Fun for Game Design by Raph Koster.

One other thing that I’ve not yet read but am interested in is Mihaly Csikszentmihalyi’s Flow: The Psychology of Optimal Experience.
It’s not targeted at games, and in fact looks at fun from a
psychological perspective, but it’s cited by most of what I’ve read so
far, and is the product of some very thorough research.