Csikszentmihalyi, Mihaly. 1997. Finding Flow: The Psychology of Engagement with Everyday Life. New York: Basic Books.

Adapted from the three following sources: See http://wp.me/p1TTs-ki
Original concept: Csikszentmihalyi, Mihaly. 1999. Wikipedia graphics @ http://en.wikipedia.org/wiki/Arousal
Plus adaption by Klingberg, Torkel. 2009. “The Information Flood and Flow.”