1. Bartle, R.: Designing Virtual Worlds. New Riders, Indianapolis (2003). ISBN 0-1310-1816-7
2. Bartle, R.A.: From MUDs to MMORPGs: the history of virtual worlds. In: International Handbook of Internet Research, pp. 23–39. Springer, Indianapolis (2010). https://doi.org/10.1007/978-1-4020-9789-8_2
3. Bell, M.W., Robbins-Bell, S.: Towards an expanded definition of “virtual worlds”. In: Fábio V (eds.), Digital Media: Audiovisual, Games, and Music, pp. 125–134. Fábio Villares (org.), Rio de Janeiro (2008). ISBN: 9788576501817
4. Benford, S., Magerkurth, C., Ljungstrand, P.: Bridging the physical and digital in pervasive gaming. Commun. ACM 48(3), 54–57 (2005). https://doi.org/10.1145/1047671.1047704
5. Brennan, S.: Redefining MMOs: Pesky Persistence. URL: http://www.engadget.com/2009/08/14/redefining-mmos-pesky-persistence/ (2009)