This is an unusual book. First of all, the in the US there is a pretty clear anti-drinking message by the church, so anything that merges drinking and God is usually suspect. The first chapter of this book does a good job giving a history of beer, civilization and the church. I think it makes a pretty good case that historically beer has been a civilizing drink, that the church did not disapprove of alcohol until fairly recently and that there is a real reason to think about how the church views alcohol.
The rest of the book is mostly concerned with illustrating, through the story of both the Guinness company and the lives of the members of the Guinness family, that people can use their secular positions to do work that they feel called to do. The Guinness company and family have been strong advocates for civil rights (in Ireland they advocated for Catholic rights from the very beginning although the family has been Protestant all along), anti-poverty programs, public health for their own workers and the communities where the company worked and explicitly Christian causes. Arthur Guinness brought and almost solely funded Sunday School to Ireland. Another Guinness was of the same era (and almost the same status in the UK) as Dwight Moody. Many Guinesses were involved in politics and banking.
There is a real story here, although Mansfield at times seems to veer into hagiography. The story seems both too detailed and not detailed enough. It is too detailed for a brief story of the family and company but not detailed enough to really prove the point of the book. For instance, there were many Gilded Age companies in the US that also cared for their workers and many company CEOs and managers that viewed their role as caring for their workers (often with a religious motivation.) Mansfield doesn’t really say why Guinness is any different than many of these other companies or how later CEOs were influenced by religious motivations.
The book is interesting, but I am not sure that it really adds much to the thinking of the Christian idea of vocation, the history of corporations or a more balanced perspective of Alcohol in the US church.