Flow in Games, Jenova Chen's MFA Thesis
jenovachen.com
jenovachen.com
Some other great literature on games that also discuss flow can be found in Jesse Schell's book (http://www.amazon.com/Art-Game-Design-book-lenses/dp/0123694...) or in Steve Swink's book (http://www.amazon.com/Game-Feel-Designers-Sensation-Kaufmann...).
(edit: no coffee yet)