The flying missile narrowly missed hitting the President yesterday.
It is not clear what the book was, where it came from in the crowd, or why it was thrown at Mr Obama - who did not appear to notice the danger.
But it is expected that there will be fallout from the security breach as the Secret Service investigates how close the President came to danger.
The rally was clearly an eventful one - other images showed a naked man being led away in handcuffs by police.
It is not clear if the man was involved in the book-throwing incident - or why he was not wearing any clothes.
The bizarre incident recalled the moment in 2008 when an angry Iraqi journalist hurled a shoe at then-U.S. President George Bush during a press conference in Baghdad.
The surprisingly nimble Mr Bush ducked the shoe - and the moment became immortalised with online parodies and internet video games.
But the incident was also marked with controversy as U.S. media questioned why the Secret Service - whose members are supposed to be willing to take a bullet for the President - were not close enough to Mr Bush to deflect the attack.
It is expected that the same questions will be asked about yesterday's incident.
Source:
Obama has the book thrown at him: Moment a missile narrowly misses U.S. President's head
