Zuckerman makes and substantiates a very specific claim: societies with low levels of religious belief are quite capable of being humane, well-run, well-organized, and can have low-levels of crime and high-levels of happiness. His case study is Denmark and Sweden. He would have found nearly identical results throughout Scandinavia, and similar results in Britain, France, Japan and elsewhere.
This is in stark contrast to the non-fact based claims of the religious right that societies need religious belief to avoid falling into brutish chaos. Statistically, the developed country with the highest level of religious belief, the US, has among the highest levels of violent crime, the least effective social safety net, the highest rate of teenage pregnancy and abortion, the lowest level of academic achievement, and so on. Clearly, prayer and reliance on a deity has not helped the situation.
Unfortunately, this is not a terribly interesting book. The various chapters consist mostly of excerpts from the many interviews he conducted during his 14 months in Scandinavia, and it all gets a bit repetitive. Most, but not all, of the people he interviewed were either atheist/agnostic, or simply had no opinion about the existence of a deity (this reflects the overall status of religious belief in Scandinavia) and gave similar answers to his questions about fear of death, the meaning of life, etc.
I would have preferred that there be more comprehensive statistics and somewhat less anecdotal evidence. There have been plenty of cross-national studies on this subject, and it would have been helpful for those results to have been provided.
Nonetheless, this is a nice antidote to the nonsense spouted by Christian and Muslim fundamentalists about the importance of religious belief.