Princip did not act alone. Clark provides exquisite detail about how he was recruited and supported by the Serbian Black Hand, and what happened to his co-assassins. The Archduke's wife was shot because she was trying to cover her husband with her body. And the Archduke was not Serbia's enemy.
Vladimir Dedijer's book The Road to Sarajevo has a lot on Princip and the other conspirators--I believe one of the men involved in the plot was still alive when Dedijer was writing the book.