It's about software engineering but also about hardware and some different kinds of design outside of IT.
From an interview about the book [2]:
> Eoin: Your new book does talk about software design in places, but it’s really about design generally, and the case studies span buildings, organizations, hardware and software. Who is the book aimed at? Are you still writing primarily for people who design software or are you writing for a broader audience?
> Fred: Definitely for a broader audience. I have been surprised that The Mythical Man-Month, aimed at software engineers, seems to have resonated with a broader audience. Even doctors and lawyers find it speaks to some of their team problems. So I aimed this one more broadly.
Brooks is also the author of The Mythical Man-Month which is often mentioned on HN.
[1]: http://www.informit.com/store/design-of-design-essays-from-a...
[2]: http://www.informit.com/articles/article.aspx?p=1600886