Susan Kare is the first icon designer of Apple, and her lecture about the origins of the icons we are using on the computer these days.
It is really mesmerizing how simple yet powerful these icons are! I agree with the tought she mentions in the video: the more abstract you make an image (like a face) the more the user can feel it as a representation of him/herself.
(Kind of the same like today’s ultra-realistic videogames versus the pixel era – in my opinion.)

Susan Kare: The Mother of Software Icon Design: