I haven't read A Farewell to Alms: A Brief Economic History of the World by Gregory Clark, but it seems interesting. It contains the intriguing idea that the Social Darwinism in England set the stage for the culture that made England break out of its Malthusian trap. Perhaps it ingrained values consistent with economic growth, in that people saw what created prosperity--bourgeois values--and emulated them, or associated virtue with these same habits. Perhaps it was eugenic?

But David Warsh's criticism seems totally focused on the implications of this kind of research, and how it may revive social Darwinism, and less enthusiasm towards charity towards the poor. His main concern with the book is that it is insufficiently generous in acknowledging other research. A fair point, but a minor one. His other main observation is that it implies giving money to the poor is not a good idea. To me, that's not merely a moral question, but an empirical one: when you give money to people, nations, groups, does it pull them up, or infantilize them? I'm sympathetic to the latter, as I see America's experience with welfare as creating a sense of victimization, a lack of urgency and initiative among those who need bourgeois values more than more freebies.

But in any case, I think for nonfiction books, the main issue should be:
1) is this book making a new, true and important point?
2) Is this book an enjoyable read?

The latter point is usually untrue if you are reading something you disagree with regardless of its merit (which would be viewed very skeptically), while even if it's old news it may be enjoyable to buttress one's predispositions. But as a professor reviewing a book in your field, the key issue is #1, and Warsh seems to think even considering it is dangerous because it offends his sense of how he thinks the world should work.