Gerry McKiernan just alerted us to a book that has a free pdf version:
Vickery, G. and Wunsch-Vincent, S. (2007) Participative Web and User-Created Content: Web 2.0, Wikis and Social Networking. Paris: OECD. ISBN: 978926403746. The free version is at http://220.127.116.11/oecd/pdfs/browseit/9307031E.PDF
This was originally prepared for the OECD in December 2006.