Zero Fail: The Rise and Fall of the Secret Service by Carol Leonnig:
That night, a large part of the country celebrated. But for Obama, the danger had ratcheted up exponentially, literally overnight. The Intelligence Division, which assessed threats to the president, immediately felt itself struggling to triage and assess a skyrocketing number of threats. Agents estimated that in the months immediately before and for several months after he took office, Obama received four times as many death threats as his predecessors—as many as thirty a day.