1. Gilmore, J.B.: Play: a special behavior. In: Herron, R.E., Sutton-Smith, B. (eds.) Child’s Play, pp. 343–355. Wiley, New York (1971)
2. Santayana, G.: The Sense of Beauty, 1896, vol. 2, Critical Edition. MIT Press (1988)
3. Suits, B.: The Grasshopper: Games, Life and Utopia. Broadview encore editions (2005)
4. Koster, R.: Theory of Fun for Videogames. McGraw Hill/Osborn, California (2003)
5. Schell, J.: The Art of Game Design: A Book of Lenses. Elsevier, Burlington (2008)