This book provides a detailed explanation of the Big Five personality model. Although the explanation of the model itself was interesting, the most valuable parts of the book were the first and last chapters.
The first chapter establishes how personality models like the Big Five are generated. Unlike models with less construct validity, these models do not start with a schema and then put people into it. Instead, these models start by asking behavioral questions and then looking for clusters of correlated behaviors. These clusters become the characteristics of the personality model. Although these models are determined statistically, it is hypothesized (though not yet verified) that the reason such clusters of related behaviors exist is because they correspond to underlying tendencies in the brain. E.g., the cluster of behaviors that are labeled "Extroversion" in the Big Five model all seem to be related by an individual's reactivity to rewards. Another interesting thing to note about this method of building a personality model is that every personality factor is a continuum. Models which try to push people into discrete buckets generally fail to have statistical validity.
The second chapter is a look into the question of why personality variation exists. Nettle makes the argument that variation exists because for every personality trait, there are some situations where being high on a trait is advantageous and some situations where being low on that trait is advantageous. Even neuroticism (sometimes called low emotional stability), which seems unrelentingly negative in the modern world, can be beneficial for individuals who live in a dangerous situation. Environments tend to change more quickly than evolution can normalize to a smaller range of variation.
The first of the ending chapters discusses the factors that determine personality. Heritability is a large factor, but explains, in general, only about half of the variation in personality. General environmental factors fail to explain the rest, but specific environmental cues can influence personality. E.g., certain types of chronic threat may increase neuroticism. Cues during gestation can affect fetal development. The final factor, and perhaps the most interesting one, is that one's own characteristics can influence how one responds to environmental cues in a way that can, over time, influence personality. E.g., someone who is conventionally attractive may get more positive enforcement when they act in extroverted ways and so their extroversion may end up larger than someone who had a similar inherited background but less positive reinforcement. (By the way, another interesting thing noted in this book is that personality factors can and do change somewhat over time. They're stable but not fixed.)
The last chapter was a look at how to live with our personalities. The first point Nettle makes is that no personality configuration is "good" or "bad". Each is what it is. That said, personalities do influence our characteristic behaviors and how we structure our life narratives. Someone with low conscientiousness is less likely to inhibit harmful behaviors such as a drug or alcohol addition. Someone with high neuroticism is likely to construct their life narrative more negatively than someone lower on that scale who experienced the same events. But personality is not destiny. Personality characteristics are interesting because they predict behaviors at a level that is well above chance, but there is still plenty of variation. As Nettle points out in a thought experiment, if someone were like you on all of the personality traits, you're likely to understand the choices they make, but there are still lots of different choices you can make. E.g., the alcoholic who knows they can't stop at just one drink could choose to drink or could choose to avoid alcohol completely. The person whose agreeableness is high enough that they often neglect themselves could choose to run themselves ragged helping others or find some way -- such as pre-scheduled time -- to take care of themselves.
All in all, I found this book to present an interesting and balanced view of what personality is and how it influences our lives.