The book discusses the practices followed and observations of 37Signals in building SaaS (restricted to webapps). It commences by emphasizing that "Less is the new More!". Less software, Less complexity, Less features resulting in "More Peace" for the developers and "More Value" for the customer. There is sufficient coverage of the other aspects like fund-raising, hiring, prioritization, customer support etc.
Each facet of building a web app receives a couple of pages of coverage and at 177 pages it is very breezy but you may have to ignore a few contradictions in the book. When the book is available for free, what's your excuse for not reading it especially when it offers the experience of building software from one of the best web development companies?