Brennan was intimately involved in the run up to the raid on the Osama bin Laden compound in May 2011.
Bergen interviewed Brennan and other administration officials for his book, "Manhunt: The Ten Year Search for bin Laden, from 9/11 to Abbottabad."
He tells the story about one particular debate on whether bin Laden could be at the compound.
"At one point the analysts said whoever is living in this compound has a dog. And of course, very observant Muslims don't have dogs. But Brennan has been on the bin Laden account himself for 15 years, and he remembered that bin Laden in fact had a dog when he was living in Sudan."
Brennan supported the decision to go ahead with the raid even though it was not certain that bin Laden was in fact there.
In a briefing after the assault, Brennan got into some hot water for implying that bin Laden was armed -- he was not -- and for suggesting that bin Laden used a human shield -- he did not.
"There was a female who was, in fact, in the line of fire that reportedly was used as a shield to shield bin laden from the incoming fire," Brennan told reporters.
More recently, the White House has been criticized by some Republicans for the leaks of sensitive information on counterterrorism operations that made the administration look good during the presidential campaign. Brennan has called the leaks devastating and vehemently denied that administration officials were involved.
The controversy over leaks, the drone program and his position on the CIA's interrogation and detention program could make for some rough moments during Brennan's confirmation hearings.
The fact he is coming from the West Wing and frequently talked about the president's views on camera could make him political fodder, Frances Fragos Townsend, CNN's national security contributor, told Security Clearance.
"John came out time and again and did a lot of press early on that angered Republicans on (Capitol) Hill," Townsend said. "Do I think Republicans will treat him not as a former career professional? He's got to expect that the Hill is going to treat him as a political person who is fair game now, rightly or wrongly."
None of the former officials and experts CNN spoke to believes Brennan's nomination would be upended by any of the criticisms of him.
Harlow thinks Brennan is a hard-working, no-nonsense type with the experience to run the agency.
"John's the kind of guy who is the catcher on the team that gets all dinged up, takes his hits but gets the job done," Harlow said.