This book follows the model of Rosenfelder's Language Construction Kit
; it's basically a worldbuilding reference guide meant to help writers, GMs, and other creators.
I bought it mostly for the portion literally referred to in the title: the physics of designing a planet with proper geological, meteorological and astronomical details. The chapter on Astronomy and Geology is actually rather short. It gives a lot of information about plausible planetology, but rather than helping you simply answer questions like "How can I design a believable desert world?" (other than the note that planets shouldn't be monoclimates) or "What if I have a planet orbiting a red sun?", you actually need to work out all the details of axial tilt, rotation, and the like, plus there is a section on designing your planet's tectonic plates. (!) If you are that sort of absolutely thorough designer who wants everything specified down to the last detail, the PCK won't leave any area unaddressed, but it's not a quick reference for sketching out a few quick details about your world.
That's the major flaw with this book: it's a kitchen sink with everything. There are chapters on designing the physical planet, creating maps, designing life forms (sentient and otherwise), societies, histories, military and religion, technology, and so on. There is a lot of salient information here, but it comes in the form of long essays and examples from Earth's history and the author's own fantasy world. There's nothing much here that you can skim quickly to answer a few questions about the world you're building; it's more like a textbook than a reference guide.
Overall, I found it pretty well written and often interesting, but Rosenfelder could have used some serious editing to chop this book down to a much more concise volume with the same amount of information. The last few chapters actually talk about illustration and 3D modeling -- seriously, there's a "How to draw people" chapter! That's 30+ pages that could have been replaced with a couple of references in the bibliography. No one is buying a Planet Construction Kit because they want drawing lessons.
Worth the money? Yes, but I was a bit disappointed as the Language Construction Kit
, while also a bit bloated, stuck more closely to its topic. The Planet Construction Kit
is very bloated, and requires a whole lot of reading to extract the nuggets of information you want.