Weston Price was a dentist a few decades ago who did worldwide studies on the links between nutrition and tooth decay. He found that people groups around the world (I think he studied people groups on every continent) who were eating their traditional diets had hardly any incidence of tooth decay, but that those who had started eating diets introduced by westerners (highly processed, lots of sugars, etc) rapidly and really drastically increased the amount of tooth decay that they suffered, and their whole facial structures began to change, as well as their overall physique and general health. The photos he took of the differences between those of the same culture eating different diets are quite extraordinary. His whole idea was to find out what those cultures who did not suffer greatly from tooth decay were doing, as compared to the kind of stuff that we do/eat.
The WAPF diet is based on the traditional diets of those cultures around the world that had little to no incidence of tooth decay, and is aimed at overall physical health and wellbeing. Price's book, Nutrition & Physical Degeneration, takes you through all of his findings. Then there are a couple of other books written by others to kind of follow up on Price's writings, which form the basis of the diet. The first is Nourishing Traditions, and the other is Eat Fat, Lose Fat.
Basically, your standard western diet is about as bad as it gets, with the highly processed nature of the food that we eat - and we're so used to it that we don't even realise the damage that we're doing to ourselves. It's fascinating stuff!