The first consideration is that books are not the best source for such a recent and evolving trend.
But in reality, the best books I can think of are only loosely related to government 2.0: they address some of its foundations and theoretical underpinning.
In this sense, I would rather recommend some old classics:
Toqueville’s Democracy in America
Titmuss, Richard, The Gift Relationship: From Human Blood to Social Policy (1970).
Salamon, L.M. & Anheier, H.K., 1996. The Emerging Nonprofit Sector: An Overview (Johns Hopkins Nonprofit Sector Series, 1), Manchester Univ Pr.
Finally, the most illuminating report on government 2.0 for me remains The Power of Information.