Foulkes is a academic psychologist, and this is a reflection, based largely on studies into Western teenagers, on how adolescence in WEIRD countries works. Coming in, I was hoping for something more focused on how adolescence has evolved, which would have required a different approach. Instead, this is an in-depth look at how teens navigate risk taking, first loves, sex, and other common new experiences. Foulkes mostly concludes that teenagers take physical risks to avoid taking too many social risks, and that much of teenage behaviour is driven by a strong desire to fit in. She also notes how powerful social, including romantic, bonds are during this period, and looks briefly at impulse control. The most interesting research was into the social network functions of high school, including the researchers' dilemma of how to describe the "popular" kids when it turns out, consistent across studies, nobody actually likes them (as opposed to actual popular kids, who tend to have social capital but not the high status of 'popular' kids).
It was readable, and surprisingly emotional - Foulkes notes how much we all have still unpacked from our own adolescence, which tends to continue to evoke shame, embarrassment and sadness - as well as joy - in adults throughout their lives. 
For action, she advocates against an approach which tries too hard to de-risk adolescence through surveillance, noting that an important developmental stage involves learning to survive challenges, trust in your own resilience and endure bad choices, as well as learning to make better ones. I just wish that had been based on a broader range of evidence.