Narrowly construed, the book is about the nature and origins of managerial rationality - i.e. the production of seriousness. However, the book is about quite much more as it discusses individual and social behavior from a multitude of different perspectives.
I think that it would not be too unfair to say that the book is about the human condition in general, with occasional more focused explanations of managerial behavior in particular.
As a very, very nutshell summary, Gustafsson appears to be quite opposed to "homo economicus" kind of thinking as overly simplistic, and instead argues that managerial behavior - as all human behavior - is quite mimetic and culturally embedded, i.e. not at all as rational as the proponents (if there any longer are any) of the "homo economicus" model would suppose.
Was it good?
Yes, the book is good - especially if one takes it from the "human condition" perspective. If, on the other hand, one takes it from the specific "managerial behavior" perspective, Gustafsson's very expansive and multifaceted discussion may be a bit too broad for this particular purpose.
I think that a scholar can not write this kind of a book at the early stages of his/her career for at least two reasons. First, this kind of a treatise requires quite broad reading in multiple academic traditions, which requires time and -- well -- reading. And second, this kind of a book is most probably so time-consuming to write that it would not be a good career move to embark on such a project at the expense of more focused (i.e. resource-efficient) publications.
The main take-away for me?
Gustafsson quite nicely illustrates how broad reading allows one to make sense of the world quite richly. Thus, treatises such as this certainly encourage one to read and study more and with a wider scope. And yes, rationality is not a good starting assumption as a "model of man", but this is nowadays not a real take-away for most people.
Who should read the book?
This is a difficult question. Perhaps a bit surprisingly, I think that management researchers - that is, modern-day career researchers - are not the prime audience of this book. Instead, I think that not-so-career-oriented scholars - in armchairs or universities - with broader horizons will enjoy the book the most, and get the most out of it.
The book on Amazon.com: The Production of Seriousness