Sunstein, C. R. (2006). Infotopia: how many minds produce knowledge. Oxford; New York: Oxford University Press.
Anything by Cass Sunstein is required reading. (For example, see Nudge, his book with Rich Thaler, reviewed on this website.) A lawyer at Harvard law School, Sunstein writes wisely about the role of groups in decision making, including the downside as well as the upside .An easy to read book about critically important topic. Groups can produce astoundingly accurate results, but they can also be incredibly narrow minded bigots, producing horrifying results: witness mob behavior in riots and lynching.