I don't usually talk about popular science books, but this one looks pretty interesting, given that I'm interested in brain size and how it may be related to social intelligence.
...and related to this is a review paper in the latest issue of Evolutionary Anthropology:
Unity in diversity: Lessons from macaque societies
Evolutionary Anthropology: Volume 16, Issue 6, 2007
Abstract: The macaque radiation is as old as the hominin radiation, approximately 7 million years. After Homo, Macaca has the widest geographical range among primates, and both of these genera are present in tropical and temperate regions as well. Whereas the single extant representative of the genus Homo diverged through processes of cultural diversification, extant species of macaques emerged through processes of evolutionary diversification. Macaque societies are characterized by profound unity and great diversity, and can best be described as variations on the same theme. To understand macaque variation and adaptation, we must take into account the processes that insure the persistence of their societies across generations and environments.