The book is essential for everybody interested in the human being.
The reasoning and the experiences that she has collected to show that peer influence is more prevalent and lasting than the influence of parents is firmly established in the book. Many have already detailed these experiences in the comments and not worth it to repeat here.
Clearly, to improve the adaptability to the environment, is much more appropriate for children "adopt" rules for living with different groups that have little relation to what goes right or wrong at home. Thus, it makes sense the idea of "code switching", in different environments.
The resistance to most people accept even listen these ideas (which therefore implies the low book evaluation in Amazon) stems from the fact that they need and want to have control over the fate of their offspring. This also stems from the deep belief and morality that parents act as a master guide for their children.
Judith Harris, on the other hand, tells that the guilt burden that parents bear when something goes wrong is too heavy and unfair.
The core goes in the right direction, but it does not prevent, to my understanding, make inferences, at least, risky.
* I personally do not believe there is a separate module for interpersonal relationships and group relationship. There is even some kind of separation, but there are important links, both direct and indirect. Direct, because groups and friends have in common and share experiences. Indirect, because close friends influence you to select groups or subgroups by affinities.
* So, although the parents did not consist of a group, the books doesn't tells that love and family care does not implies identification and sharing experiences. Thus, for example, a very close relationship between a mother and a daughter who goes beyond the love and care, can direct the child to adopt behaviors and attitudes that may affect the way she faces the differents groups. No reseach is described in the book explores this possibility, which is lost in big and generic samples
* On the other hand, I think the issue of "abuse" is handled too much generically. Families of thieves or drunks does not mean that abuse continued happen to their children. The author even addresses this issue lightly, but not much stands out. All research that uses it quoted a very broad universe of maladjustment. If one single rape can cause sequelae in adults why a child continually mistreated will not be affected? Further research would need to be made on this issue.