I'm very conflicted about this book. in the abstract, it's a fascinating study of human cognitive evolution as a continuous function -- though of its accuracy I'm not entirely convinced. In hindsight, it appears to explain a huge number of phenomena from my past relationships which at the time I considered almost inherently mystifying -- however, the strength of a theory is not how well it fits the past, but how well it predicts the future.
Unfortunately, Kegan seems almost enamored with Freud, and attempts to fit as many of his own models to agree with Freud's. Kegan's lack of skepticism in this regard strikes me as ominous; I can only wonder how much skepticism he has applied to his own models. While this not an explicit reason to disbelieve Kegan's theory of cognitive development, it is certainly sets off loud alarms. Most of the book's arguments come anecdotally, with a startlingly small sample size -- the majority of the book focuses on only three individuals, though Kegan says the theory itself is derived from "interviews with over 40 patients". Perhaps most damningly, the book relies *far* too heavily on large, incomprehensible tables spanning multiple pages with no visible signs of organization.
Though he does not formalize it as such, Kegan's theory seems to implicitly model human cognitive as a continuous oscillatory function, mapping from time to an axis of ego-differentiation/integration. Kegan states that these are opposite sides of the same coin, and strongly suggests that the ideal balance is the equilibrium between the two. The book offers some actionable advice on how to inspire transition between the cognitive stages, and how to notice the transition when it occurs. Furthermore, it suggests the reason that we are sometimes completely unable to see others' arguments is that they are aimed at a level we are not able to comprehend, let alone appreciate.
The final section of the book consists of advice for psychologists; it is entirely skippable for those of use who are not professional psychologists, and, though I am not an expert in the field, I would suspect it is indeed skipple for everyone entirely.
In conclusion: if you're interested in this book, read the Wikipedia page instead. You'll save yourself a lot of time and headache.