By Edward Conlon. Riverhead. 562 pp. $26.95

With cops, it's hard to tell where the person ends and the job begins. Most off-duty officers can't shake the hard edge of watchfulness, and one gets the feeling that they go through life dividing the good guys from the bad. A cop, it seems, is always a cop, and outsiders shouldn't even try to understand.

In Blue Blood, his memoir of life in the New York Police Department, Edward Conlon would seem just the man to keep his two worlds apart. Harvard-educated and a gifted writer, Conlon has been contributing the "Cop Diary" to the New Yorker under the name of Marcus Laffey. But anyone expecting a neat separation between officer and writer will be disappointed. Conlon is a cop's cop and his book, a dazzling epic of street life and rough camaraderie, is far more rewarding than any disgruntled Serpico-style tell-all could ever be.

Conlon resisted becoming a cop, though in retrospect it seems inevitable he'd fall for the siren song of law enforcement. His father was an FBI man; numerous uncles and family friends walked the beat. Perhaps as a type of rebellion, Conlon became a low-grade hooligan who believed that "cops were firm and fair and mad at you, a lot of the time, for good reason." After straightening up and completing college, he worked in a program designed to steer "good" convicts toward the mainstream. Soon, though, he realized that this desk-bound relationship to criminals didn't offer the thrill he craved, and he entered the police academy.

In relating his life as an NYPD officer, Conlon thankfully avoids flogging broad agendas. Instead he immerses the reader in his blue world as he crashes through doors and cajoles junkies into giving up information. Although he eventually is promoted to the rarified air of the Detective Bureau, he revels in the ground-level action of "buy and bust" narcotics work. "When you hit a [drug deal], there is always a charge of adrenaline, arising from the jungle-war vagaries in your knowledge of the terrain and the determination of your adversary. . . . In brief, it could be a surrender as slow and dignified as Lee at Appomattox, or it could be bedlam, a roil of running, struggling bodies, and airborne stash." Conlon has an ear for the cadence of the projects, and his use of slang and dialogue is masterful. He laments that he must prettify his hard-won ghetto language to fill out a report on a drug deal, wishing instead that he could write "to wit, defendant did possess one mad fat rock of yayo." The verbal sparring between partners is also well rendered, and the men he works with -- guys with nicknames like Smacky, Pops and the Short-a-Rican -- are vibrant and hilarious.

A reader looking to criticize the culture of police work would find plenty here that is offensive. But the writer is a good and caring cop, as are the people he works with. So what if Conlon, an Irishman, and his partner Timpanaro, an Italian, compete to see how many of their countrymen they can arrest in a good-natured game they call, with bureaucratic perfection, "Mickstat and Wopstat." And is anyone really hurt when he describes the protracted arrest of an uncooperative prostitute as "Operation Lying Whore"? Impolitic to be sure -- but Conlon isn't trying to win any admirers on the civilian review board. He's just trying to be a regular cop, and an honest writer.

More important, Conlon recognizes the legitimately sensitive situations his profession forces him into. He regrets that a serial woman-beater, for example, goes back on the street because the man is an integral part of another ongoing investigation. When an informant offers a tip about a hidden gun, the money he's paid will probably go back into drugs, and eventually toward a new gun. The net gain isn't quite zero, but sometimes it approaches that number, and Conlon is a realist about his chances of staying ahead of the criminal element.

Conlon also feels real sympathy for the people he encounters. He sees a shadow of himself in a twitchy, drug-addled informant he has cultivated, and when he writes that their meetings have "the affectionate but awkward quality of a divorced dad picking up his kid every other weekend," the words are honest, with none of the self-conscious big-heartedness that civil servants often profess.

If there is a drawback to this fascinating ride-along, it is that the narrative hews too closely to the trajectory of Conlon's career. Long pages are devoted to settling scores with loathsome supervisors, and when he describes weeks spent doing nothing more interesting than parking-lot duty at Yankee Stadium, the book drags. Still, it is reassuring to know that the world is occasionally peaceful enough for a cop to endure maddening stretches of boredom.

The last decade must have been a confusing time to be a New York cop. The city is undoubtedly safer than it has been in years: Gone (or at least subdued) are the fare-jumpers, the panhandlers and the dreaded squeegee men. But this renaissance has been dogged by gripes about thuggish police work and suggestions that civil liberties have suffered. More poignantly, the ultimate sacrifice made by many of New York's finest on Sept. 11 sits awkwardly alongside the tragic mistake that led to the death of Amadou Diallo and the depraved abuse of Abner Louima.

Blue Blood doesn't attempt to sanitize an entirely human institution. Instead, Conlon presents the truth as he has lived it. He is no outsider casting stones, but the ultimate insider, a man so committed to his work that he takes his partner as his roommate and chooses, for his sole off-duty pastime, to write movingly about his long days on the job. *

Zac Unger's memoir "Working Fire: The Making of an Accidental Fireman" has just been published.