Clusters are often approached by economists and political scientists from a policy perspective. Here is a social science perspective using networks.
Research from Jason Owen-Smith (eg.2004, 2005, 2009 papers) has always been very exciting. Here he talks about clusters and their inner mechanics in respect to demographics of talent and technology.
(Additional Info: TRE Networks Roundtable and Annual Meeting, Washington, D.C., Dec., 5-7, 2012)