In Szeliski book I find more and more interesting references.
For example a paper by Leyvand, Cohen-Or and Dror that describes how to warp a portrait into a "more pleasing" portrait by moving features according to images that were rated as being beautiful.
And in my subjective impression it is working quite well. The modifications are quite subtil but make a big difference on the overall impression.
I don't really want to think about the implications of the existance of such an algorithm... but it is quite interesting anyhow.