I tend to think of this as a companion book to Brumberg's Fasting Girls, which is a history of the development of anorexia nervosa as a disease and a look at societal factors that contributed to that as well. The Body Project is a closer and more specific look at American girls in the nineteenth and twentieth centuries. Brumberg mostly investigates how girls' body images have changed from the Victorian period to the present day, emphasizing that while the Victorians shaped the body with corsets and constrained it sexually, people of today shape the body via diets and put pressure on it sexually.
Brumberg looks at several factors involved with girls going through puberty and how they've changed, including attitudes about menstruation, acne, tampons, virginity, and training bras. I had some vague knowledge about some of the things that Brumberg writes about, but it's still interesting to see what Victorian gynecologists did when they had to conduct exams. Apparently many of them had never conducted an exam of a girl's genitals until she was married, sometimes only after she was a mother. The acne bit also amused me, as acne was apparently a sign of sexual thoughts.
This is rather sloppy generalizing on my part, but her conclusions largely seem to be that while girls' knowledge about their bodies and reproductive systems increases through the century, their acceptance of themselves may not. Brumberg also notes the increasing influence of business and advertising on these supposed rites of puberty (I'm not much of a fan of the whole "Oh you're becoming a woman, how sweet!" thing myself); tampon and pad manufacturers putting out pamphlets to explain menstruation, as opposed to girls learning from home, advertisements for the necessity of training bras to develop the "proper" figure, advertisements for pimple creams intended to make people feel bad about their acne.
I do partly sympathize with Brumberg's point; there is a giant industry out there that wouldn't be making quite as much money if suddenly everyone decided that they looked just fine and didn't need whitening creams, Clearasil, push-up bras, etc. On the other hand, Brumberg seems to advocate the involvement of mothers in their daughters' lives. Part of this does seem good, especially in the chapters where Brumberg is emphasizing the increased pressure on girls to act sexually mature at younger and younger ages. On the other hand... I would have died before telling my mom anything. I'm not saying that parents don't have lots of influence, since they obviously do, but I think it shouldn't just be limited to the parents, especially during adolescence.
In the final chapters of the book, Brumberg sort of summarizes the constrictions upon girls in the Victorian era and how they loosened through the century, while newer, different rules were made. Luckily, she never quite falls into the trap of blindly longing for a golden age of the past; she's smart enough to realize that it's not optimal to keep all knowledge about sex and the like from girls. She does think that a culture that encourages and pressures girls to have sex earlier and dress sexily is harmful, not because of the modesty factor or that no one should, but because she feels that many girls are doing so because of peer pressure and not because they're making informed choices. Mostly, she advocates protecting girls by educating them rather than shielding them from knowledge, by helping them feel comfortable and confident about their bodies and their minds, which is something I very much agree with.