The Flat Belly Diet is a massively popular book written by Liz Vaccariello and Cynthia Sass with the intention of helping women flatten their tummy.
The marketing for this book makes the following claims:

• Losing up to 15 pounds in 32 days
• Not requiring any exercise to lose belly fat
• Hardly any nutritional deprivation
• Most of the fat loss will be centered in your belly.
  

The Flat Belly Diet advocates eating a lot of Monounsaturated fats both as a way to avoid unhealthy fats and because the authors make the claim that this fat can actually contribute to abdominal fat loss.
Whether this is true is a topic of debate.
 The book does include reference to a study which supports this claim, but other health professionals have opposing views.
It is always a bit hard to believe that eating some specific food can help you lose weight, especially if it's more from a specific body part, but the Flat Belly Diet has many positive reviews from women who've read it, so the results may surprise you.
Now, it's true that you can lose all the weight that you want without doing any exercise since nutrition is the most important thing.
However, I do recommend exercising for faster and longer lasting results, and the Flat Belly Diet Book does have a segment on exercising even though the authors claim it isn't necessary to flatten your abs.
The diet is made up of 2 parts: 1.
A 4 day initial phase which is rather low on calories and is intended to help you get rid of abdominal bloatiness, which alone can make you look much leaner.
2.
A 28 day phase in which you eat 1,600 calories a day and concentrate on eating a lot of Monounsaturated fats in each and every meal.
The entire diet lasts for 32 days, after which I urge you to make a routine out of working out regularly and eating properly to make sure that your results last a lifetime.
Other than that, the book is well written and has excellent reviews.
It's worth a read even if you eventually decide not to use it yourself.