R. Jäschke, A. Hotho, F. Mitzlaff, and G. Stumme. Recommender Systems for the Social Web, volume 32 of Intelligent Systems Reference Library, Springer, Berlin/Heidelberg, (2012)
B. Sigurbjörnsson, and R. van Zwol. WWW '08: Proceeding of the 17th international conference on World Wide Web, page 327--336. New York, NY, USA, ACM, (2008)