If you want a primer on Wilkinson and Pickett's joint book The Spirit Level, then the pieces here are worth a look (one by me). And for a comprehensive set of responses to their critics, including a pre-emptive strike against Gerry Hassan's recent piece on OK this is all you need. (It is worth noting too that Wilkinson and Pickett's work is peer-reviewed; that of their critics isn't.)
For me as a philosopher, the thing about The Spirit Level that is most exciting is that as a study of the pervasive harms of inequality it strongly suggests that John Rawls's 'difference principle', which says that inequalities are OK provided that they materially benefit the worst off, a principle that has dominated political philosophy for 40 years, is simply wrong. Empirically wrong.
Which means that put into action 'the difference principle' will create a worse society, across a whole index of measures. Perhaps surprisingly, it will make virtually everybody, and certainly the worst off, worse off. (Or at least: worse off than they could be if an alternative way of 'organising inequalities' a more egalitarian way - were settled upon.) Even if they have more money or more things (are 'materially better-off'), this will not translate into an improved quality of life: on the contrary.
In sum: it is now possible for the first time to show that the difference principle (and, by extension, liberal political philosophy whether or not of the 'trickle-down' variety) makes the worst-off on balance worse off, and this can I think reasonably be taken to constitute an empirical refutation of the claim that it could possibly be just. (We philosophers don't often get to make empirical refutations of others' claims, so this is quite exciting!) Wilkinson has said to me, by the way, that he agrees with this reading of mine, that his and Pickett's work has a devestating impact upon the centrepiece of Rawls's liberal political philosophy.
For me as a Green, what is so welcome about this book is that it provides a powerful way (additional to our standard points about sustainability) to argue back against those who claim that the answer to the problem of poverty is always economic growth. For economic growth that grows inequality will only increase relative poverty. And this brings out once more the truth in an old idea that should never have gone away. That the main reason that the poor are poor is nothing to do with their own alleged inadequacies. The main reason that the poor are poor is simply that the rich are rich.
The Spirit Level is reviving egalitarianism powerfully at a time when neoliberalism had had its way for too long. With the vast financial and ecological crises of recent years neoliberalism was almost asking to be replaced. I can't help but see the book in this light, as a hugely valuable contribution to our political culture.