A solid foundation to the field, both for the reality that systems are everywhere and the method on how to tackle this
My initial interest in the book and this field came from reading Ludwig von Bertalanffy's General System Theory. Although in retrospect, reading Principles of Systems Science first would have been better.
The authors of Principles of Systems Science practice what they teach, often switch between fields, effectively showing how economics, biology, business, and myriad other subjects can all be seen through the lenses of Systems Science. This is part and parcel of Systems Science. Bertalanffy does the same thing in General System Theory. I personally like that, it's the way I think, so it's quite natural to me even though I'm not well versed in some of these subjects (notably biology), but to some it might be off-putting.
While the book does not require any specific advanced math, any math is well identified in the Quant boxes, there is frequent use of Graph Theory.
Also a number of chapters rely heavily on Evolution, while again no preexisting knowledge is required to understand the book, already knowing of the concept of natural selection goes a long way in keeping up.
In its aspect as an introduction to the field the book is successful. As the title indicates, it's a textbook for the principles of Systems Science. Each Chapter has a reasonably sized, but always effectively curated bibliography pointing the way to learn more about a specific topic.
The most effective part of the book is as a way to learn to think in a different way, thinking in terms of systems, instead of one to one cause and effect. Now I find that I see systems everywhere, I'm taking that as an indication that reading the book is working.