Eating too much fat is not the only thing that will make you put on the kilos. Excessive amounts of alcohol, protein and carbohydrates can all be broken down and converted into fat. The bottom line is: if you take in more energy than you burn up, you'll put on weight.
I'm not sure that is an absolute, ratio of protein/fat/carbs is important too. I think it was shown in the 50s that you do not necessarily utilise all the energy in your food (1). Hmm, I'm sounding like a proponent of the Atkins diet, which I'm not.
Here is some interesting reading from the
BMJ
Plenty of quackery out there when it comes to diet books, there is too much money involved for the snake oil salesmen to stay away.
1. Kekwick A, Pawan GLS. Calorie intake in relation to body-weight changes in the obese. Lancet 1956; ii: 155-160.