While searching for universal concepts of human beauty is a challenging business, it seems most cultures find common ground when it comes to causes of attraction. An internet search on the subject confirms that physical attractiveness is generally more important to males than females. We also tend to value symmetrical faces more highly than asymmetrical ones, so there may be some truth to the textbook's assertion that people are more inclined to "average"-looking faces.
One of the most fascinating findings to me is the notion that people are generally attracted to those with similar features as themselves. This may also shed light on the "proximity" principle on guiding interpersonal attraction discussed in the text. By looking in the mirror often, perhaps we create a certain "proximity" to our own image that makes us naturally inclined to our own looks. So maybe people are more confident in themselves than they would like to lead on (or maybe they are already vocal about their smug narcissism).
At the same time, beauty is far from being perceived as the "same" across all cultures. Masai women of Kenya, for example, are known to shave their heads and remove two of their lower middle teeth to attain such radiant beauty.
Beauty is, overall, impossible to define completely. If beauty is anything like pornography, however, perhaps most people would apply the same definition to beauty that former Supreme Court Justice Potter Stewart did to pornography: "I know it when I see it."