Olivier Corneille & Charles M. Judd 
Psychology of Group Perception [PDF ebook] 

This groundbreaking work by leading social psychologists, who have all contributed in important ways to the psychology of group perception, focuses in particular on three interrelated issues: (1) whether groups are seen to be diverse or relatively homogeneous; (2) whether groups are seen as real and stable or only transitory and ephemeral; and (3) whether group membership derives from some essential quality of the members or rather is based on social constructions.