In Al-Zaidi's case the cultural significance of shoes as an insult may have factored in, but I'd certainly agree that the subsequent shoe throwers are copy cats and could see the addition of books being simply a matter of convenience. Occam's razor certainly favors that explanation.

Ooh, now part of me really wants to see someone throw a (dairy) pie at Marzel, preferably just after he's had a meat meal! The treif aspect is innovative (if obnoxiously antagonistic), so the media would even have a reason to call it innovative! Okay, maybe not so much. Is anyone actually calling the Stockholm incident innovative?