There are a lot of weight loss diet books out there and in reality diet plays an important role in your weight. There are mostly two reasons why you gain weight: one is because you have a low metabolism rate and two is because you are eating too much and eating bad quality foods.
Having a low metabolism rate means that your body does not use much energy to keep it running which means that what you eat even if it is not much will remain because your body does not consume so much energy, that source of energy will be stored under the form of fat leading to weight gain.
Eating too much and eating bad quality foods will lead to weight gain for two reasons: one is eating too much food (quantity) which means that you'll have more calories in your body than you can burn (unless you exercise) leading to the storing of such calories under the form of fat, two is eating bad quality foods like overcooked animal oils which will lead to a increase of fat in your body.
As you can see the diet interferes directly with our weight and does so our metabolic rate. To lose weight effectively you must play with these two very important factors.
A weight loss diet should made of the smallest portions possible to maintain you satisfied until the next meal with a small lunch in between like a piece of fruit. The meals should be made of certain foods, some foods will more likely make gain weight (like saturated oils and fat), I can not explain a whole diet here but you can find a lot of information in literature or the internet, most weight loss diet books are written by experts but for weight loss I personally enjoy diets based on fat burning foods, these diets together with exercise are an excellent solution to lose weight.
To increase your metabolism you must exercise and while you exercise your metabolism will be high enough to burn calories under the form of fat. Attention that not all exercise makes you burn fat, some exercise will even make you gain weight, heavy and fast exercise will make you gain weight through the production of new tissue and storing of other substances. To lose weight you want moderate exercise which is the one which runs in an oxygen based state where fat can be totally burned into energy, this will make you lose fat. Going for a long fast walk going up a slope of at least 5% for a long time (2 hours) everyday will burn some fat, also running everyday one hour at a moderate rate of effort will retrieve its energy mostly from fat. This lifestyle together with a weight loss diet will certainly make you regain the weight you wanted plus it will keep you more wealthy and protected from possible health problems due to lack of exercise.