Kobsa, A.,  
"Generic User Modeling Systems",  
The Adaptive Web: Methods and Strategies of Web Personalization, Berlin, Heidelberg, New York, Springer Verlag, pp. 136-154, 2007.
 Scacchi, W.,  
"Game-Based Virtual Worlds as Decentralized Virtual Activity Systems",  
Online Worlds: Convergence of the Real and the VirtualOnline Worlds: Convergence of the Real and the Virtual: Springer, pp. 225-235, 2010.