[Update:] See some related thoughts about the subject of why Japanese people in Japan don't get that fat here .
I stumbled upon Japanese Women Don't Get Old or Fat  when I was browsing around Amazon some time ago. When I first read the title, I laughed. It seemed like a quite obvious attempt to cash in on the success of French Women Don't Get Fat: The Secret of Eating For Pleasure . Also (and this applies to the French Women book too) it makes such a sweeping generalization right there in the title, which borders on the ludicrous. But I was curious about it so I decided to get it for the sake of research.
The short review: it's not as bad as I thought it would be, and may serve as a good introduction to Japanese cooking, though definitely not the best.
The diet and weight loss points made in this book could be summed up in a few bullet points:
Obviously, it is not necessary to eat Japanese food to accomplish these goals. But I do agree with the point made in this book that traditional Japanese food is inherently healthy. As you probably know if you have been reading this site, I am Japanese but have lived most of my adult life in other countries. So my everyday cooking is a mishmash of various styles. If I could afford to in terms of both time and money though, I would cook and eat Japanese style most of the time.
Traditional Japanese food centers around rice (gohan, which is also a synonym for a meal), with small portions of okazu, savory things that go well with the plain rice. A typical Japanese dinner would have, besides the rice, 1 small bowl of soup (miso or clear), 1 protein dish such as grilled fish, and 2 or 3 other mostly vegetable-based side dishes.
A popular dietary guide in Japan that has been around for decades is to try to eat 30 kinds of food items a day for nutrional well-roundedness. This may sound impossible, but in Japanese cooking it's not that out of reach.
If you go to France, most women are not overweight. Many, especially in Paris, are in fact skin-and-bones slim. What may be less known is that here in Switzerland too, there aren't many overweight people - though older women tend to look more sturdy and well-muscled. Whenever I go to the U.S., or for that matter to the U.K. too, I always get a mild shock when I see the number of very overweight people. You simply don't see many of them here.
Swiss food is a mix of French, German, Italian and native Alpine - featuring loads of butter, cheese, bread and potatoes. Veal is the most popular meat, especially for company (chances are if you are invited to a Swiss home you're going to get veal in some form). So why aren't more Swiss people overweight?
The answers I think are the usual: moderation and exercise. Portions here are quite small compared to those in the U.S. The delicious pastries available at Sprüngli are barely bigger than my palm, and their handmade truffe du jour is so rich that one (and yes, they do sell them by the piece) is enough to satisfy any chocolate urge.
Swiss people also exercise a lot. Whenever we're invited to someone's house, invariably we go for a walk after dinner to stretch our legs and to see the neighborhood. The house I live in is on a corner, and I always see couples and families walking or biking past on the weekends. And of course a lot of people participate in winter sports as well as hiking and camping in the summer.
What's the typical after-dinner-party activity in the U.S.? Watch TV, play video games, or just sit around and chat and snack some more?
Getting back to why Japanese women aren't overweight: I discussed this with some Japanese people and we all seem to agree that in Japan, there is a lot of societal pressure to stay slim. Young girls often starve themselves so they can be fashionable. Older women are also quite pressured to stay thin. The standard clothes size in Japan is junior size 11, which is about a size 6 in the U.S. Japan is inherently a comformist society, so it's difficult to 'stick' out in any way, including being overweight.
As far as Japanese women not getting old - they do of course. Do they look younger than their Western counterparts? Often yes, but I think that all Asian women tend to look younger than European-Caucasian women. I still get carded in American bars and I'm nowhere near 21 anymore. As far as the various illnesses related to getting older, while the incidences of heart attacks and diabetes may be low, there are other problems such as a high incidence of osteoperosis amongst older women.
So to wrap up, I would recommend this book if you want once with some nice recipes (though without pictures) of some basic Japanese foods, with some diet advice along the way. But don't buy into the premise that there is some magic aspect to Japanese food. If you overeat it, you're still going to gain weight! The main lesson to be gleaned from it is hara hachibunme.
See also: Why Japanese people in Japan don't get that fat