The thesis of the book, as the author states at the beginning:
under the right circumstances, groups are remarkably intelligent, and are often smarter than the smartest people in them. Groups do not need to be dominated by exceptionally intelligent people in order to be smart.
The book’s web site: http://www.randomhouse.com/features/wisdomofcrowds/
Wikipedia page on the book: http://en.wikipedia.org/wiki/The_Wisdom_of_Crowds