The glycemic index is totally useless for dieting unless you are accustomed to eating just one food item per meal. The glycemic index provides the the rate at which a carbohydrate converts to glucose, enters the bloodstream, and causes an insulin response. Unfortunately, this is flawed in two ways.
1. Some food items, for example carrots, have a very high glycemic number, but they are so low in calories that, in order to get a blood sugar spike, you would have to literally eat an entire bushel of them.
2. Most normal human beings mix their proteins, carbohydrates, and vegetables during a single feeding, and proteins and fats will slow down the absorption of glucose into your bloodstream. So unless you plan on being a "one trick pony" at every meal, forget about using the glycemic index for dietary purposes.
Like I said SPRING FOR THE BOOK!"