Harmful. In fact I'd go as far as to say most dieting books are potentially harmful regardless of their target audience's age. A proper diet is one that's balanced around what you need, not want and no book can say exactly what you need to lose weight without the author examining you personally and custom writing a book.
These books also put immense pressure on people to conform to a weight standard that's almost arbitrary and fails to take into account virtually anything about a person's physique, metabolism, height, exercise level or capability or any other possible contributing factors to how weight gain and loss are achieved.
If you want to diet, here's all you need to know, in a general sense. Stay away from excessive amounts of anything unhealthy (salt, fat, sugar, etc). Focus on vegetables, fruits and exercise. Meat and junk food in low moderation and don't let anyone tell you how much you should weigh unless they're a qualified doctor or nutritionist.
Pretty much common sense stuff we all forget or never learn because schools fail at physical education. Children should only be on diets if they truly are obese and follow a diet from an aforementioned expert, not some book that tells you something without knowing anything about you.
