Not to discourage further posts which might help answer your question, but Top Quality Contributor /u/mikedash wrote an excellent article in the Smithsonian magazine in 2011 which suggested that the idea that Gavrilo Princip was eating a sandwich comes from a 2001 novel and/or a 2003 BBC documentary, and doesn't seem to appear in many sources previously; also, Schiller's delicatessen was on the route that the Archduke Ferdinand was taking.