Walter Benjamin famously asserted that "that which withers in the age of mechanical reproduction is the aura of the work of art". In his terms, the aura is that which is original or authentic about a work. Aura depends on the position of a work within a tradition and its uniqueness. Reproduction diminishes each, the argument goes.
I was looking at Books and culture [scanned copy] [find in a library] in Google with a colleague earlier. It was nice to see the 'auratic traces' left by the staff at the University of Michigan on the scanned copy - the cataloger's marks, the UM perforated stamp. Maybe we will see books emerging with annotations, autographs, coffee stains: that would be good.