The book covers 25 different open source projects, some of the most notable being: CMake, Eclipse, LLVM, Sendmail and Berkley DB. Each chapter stands on its own and is written by the project lead or one of the lead contributors, while following a loose set of questions such as "how did it start", "what would you do differently", and so on. All in all, a fun an educational read. Some chapters go into the nitty gritty technical details, while others stick to the high-level architectural view.
As intended by the title, I think the most valuable part of this book is simply reading about the history and lessons learned from all of the different projects. The authors don't provide any conclusions, but you can easily spot patterns as you make your way through the book.