Kerik, 46, has wielded that authority reassuringly in the wake of the terrorist attacks. When he took over the department a little more than a year ago, few expected him to do much beyond closing out Mayor Rudy Giuliani’s term as peaceably as possible. Instead, he’s had to steer a force of 41,000 that is stretched thin as it confronts a brave new world of security threats. In the eyes of many, he’s performed well, deploying cops to meet a host of new policing demands while also reducing crime by 13 percent so far this year, compared to the same period in 2000. It’s hardly surprising that Michael Bloomberg, the mayor-elect, urged Kerik to stay on the job (Kerik declined on Friday, preferring to explore private sector opportunities instead).

It would have been a coup for the new mayor, whose police department faces challenges on multiple fronts in the months ahead. Officers are investigating the possible existence of terrorist cells in the city. They’re trying to piece together the puzzle of Kathy Nguyen’s death from anthrax, which Kerik says is “still really a mystery.” They’re beefing up security everywhere from ball parks to tunnels. They’re dealing with the usual array of robberies, rapes, and murders. And they’re doing it all amidst a welter of emotions spawned by the loss of so many lives in the attacks.

Tempers flared recently when firefighters protesting the reduction in personnel at Ground Zero scuffled with cops; 18 firefighers have been arrested so far. Demonstrations are “fine and dandy,” says Kerik. “But you can’t slap cops around.” One thing making life easier for police: they’re more likely to be showered with gratitude these days than accusations of brutality. “It’s been an enormous morale booster,” says Kerik.

As trying as this year has been for Kerik professionally, it’s been just as wrenching emotionally. At about 1:30 a.m. on Sept. 11, he completed the final chapter of his autobiography–a chronicle, in part, of the toughest investigation of his life. In researching the memoir, Kerik delved into the past of his biological mother, Patricia Kerik, who abandoned him at age 4. His discovery: that she was a prostitute involved in a white slavery ring; that she probably turned tricks in Times Square, where years later Kerik would patrol and routinely bust prostitutes; and that she was likely brutally murdered in Ohio–a death that was never investigated since a white woman consorting with black pimps in the 1960s didn’t arouse much concern. With each new revelation, Kerik thought, “Enough, Holy Christ … Just put me out of my misery.”

Kerik and his investigators never solved his mother’s suspected murder and probably never will. Though he believes he’s narrowed the list of suspects to three unsavory characters, they’re all dead. But his sleuthing has granted him a different sort of closure. For one, Kerik has replaced the bitterness provoked by his mother’s memory with forgiveness. And he finally figured out what prompted a nightmare of abandonment that has haunted him for 40 years. Upon that discovery, “I broke down,” Kerik recalls. “It was this amazing feeling to finally know what [the dream] was.”

Kerik also learned that history has a cruel way of repeating itself. As an MP in South Korea in 1975, he had a baby girl named Yi Sa (Lisa) with a Korean woman. When he was transferred to Ft. Bragg, N.C., he left them behind. Though he later searched for them, he never found them. “Here I was, doing to an innocent child something very like what had been done to me,” Kerik writes. Only this year, however, did he realize it. “I was an immature, stupid kid,” he says.

Kerik had a rough time growing up. He lived in a poor neighborhood in Paterson, N.J., fighting frequently and eventually dropping out of high school. He always wanted to be a cop and in 1987, he finally joined the NYPD. Kerik rose through the ranks quickly. Donning earrings and a ponytail, he became a star undercover narcotics detective who brought down members of Colombia’s Cali cartel. Later, as commissioner of the Department of Corrections under Giuliani, he presided over a 93 percent drop in stabbings at the notorious Rikers Island jail.

When Kerik became police commissioner, plenty of officers harbored doubts about a guy who never rose beyond detective. So in his first meeting with his command staff, he recounts in his book, he told anybody who had a problem with him to “get over it.”

They did.

The publication of Kerik’s memoir–“The Lost Son,” due in bookstores on Tuesday–has already sparked controversy. Some critics have suggested that he’s profiting from tragedy with the addition of an afterword on the attacks and his use of 32 pages of police photographs. Kerik’s reply: that he “did nothing on city time” and that he’ll donate a portion of the proceeds to a victims charity.

Soon Kerik won’t be on city time at all. He tearfully announced on Friday that he’ll be departing with Giuliani on Jan. 1. He leaves behind a police department struggling to balance its new mandate to prevent terrorism with its usual duty to fight crime. But Kerik is confident that the department will rise to the challenge. They certainly did on Sept. 11, conducting what he calls “the most successful rescue mission in the history of this country.” Not a bad legacy.