BLUE BLOOD
By Edward Conlon
562 pp. New York:
Riverhead Books. $26.95.

EARLY in this sprawling, wry, opinionated, beautifully written memoir, Edward Conlon writes that he laughs a little whenever editorials denounce police power. To a patrolman, he says, the job doesn't seem very powerful. Charged to embody lofty ideals, cops are jerked around by politicians, superiors and just about everyone else; they are mistrusted from within and without. He might have spent ''hours on a frozen rooftop, watching below,'' only to have a supervisor on rounds ''reach out and feel my shield, to see if it was cold to the touch.'' When police officers are sick at home, he notes, they have to call for permission to leave the house, and check in again when they return. Though vested with the power to use deadly force, cops have been treated like schoolchildren. These contradictions, Conlon says, have led many New York City police officers ''to develop a decidedly ironic point of view.'' And it's this, he maintains, that best explains the department's ''blue wall of silence'': ''It's not so much that cops don't want to talk, it's that they can barely begin to explain.''

Never has a cop explained like this -- and a working cop, at that. The New York Police Department has, of course, inspired a huge variety of popular entertainments over the years, from genre novels to films and long-running TV shows. But ''Blue Blood,'' in terms of its ambition, its authenticity and the power of its writing, is in a class by itself. Conlon is uniquely qualified to write about this giant (four times the size of the F.B.I. when he was hired) yet famously insular tribe. Among the city cops in his family tree are his father, uncle and a great-grandfather, a crooked policeman who used to ''carry the bag on Atlantic Avenue.'' His family is Irish Catholic, solidifying his insider status, but at the same time Conlon is an outsider: after a Catholic education in the city, he went on to Harvard.

The story begins with Conlon, a rookie at 30, on foot patrol in the South Bronx. He likes the ''spontaneity and variety, reacting to the rhythms of the street, with its long lulls and sudden convulsions.'' The neighborhood's streets and buildings are the book's canvas, the backdrop to smart observations (''when you arrest someone, it's like a blind date''). We meet a multiethnic cast of other patrol cops and Conlon's extended family, and learn about his own brief dalliance with delinquency. We flash back to training at the academy. Then it's a return to the South Bronx, where he joins the plainclothes Street Narcotics Enforcement Unit, and, some time later, becomes an investigator with Bronx Narcotics. Conlon, who says he became a cop ''precisely to avoid work that entailed a suit, a commute and a cubicle,'' has a scare when he is nearly made an assistant to Police Commissioner Bernard Kerik after the commissioner hears about his education. But that falls through, and soon he makes detective.

''Blue Blood'' runs from the episodes of a cop's life to meditations on that life, from gun-in-hand assignments like executing search warrants to mundane calls about vicious cats. Layered with Conlon's family history; allusions to St. Augustine and early police books; and his own views on departmental dysfunction, the war on drugs and urban race relations, the book becomes a kind of rich ethnographic document, steeped in what Clifford Geertz might have called ''thick description'' (not to mention a bible for future generations of cop show writers). There aren't many dates or other signposts here, though, and not much narrative arc; it's easy to lose the way. Conlon is funny on the subject of police language (''the general oddity of cop talk, its shotgun marriage of street slang and legalese''), but woe to the reader who, by Chapter 12, forgets the difference between a P.O. and an O.P., the 61 and the five, a bag and a skell. You might want to take notes as you go.

Similarly, given how much he does say about his extended family, his omissions regarding his personal life are curious and sometimes frustrating. We learn he's the second-oldest son only when he wonders if second-oldest sons have a problem accepting authority, and find out he has a girlfriend only when he's dreading going back to work after spending time with her. Nothing is revealed of his Harvard years except that he got in a fistfight and was placed on disciplinary probation soon after arriving. And on Sept. 11 we discover, out of the blue, that two of his brothers work downtown, one as a lawyer and one as a banker, but only when a worried Conlon admits he doesn't know exactly where either has his office.

Some of Conlon's most absorbing digressions concern episodes in the life of the modern New York Police Department, such as the French Connection heroin case; the corruption cases involving Frank Serpico, David Durk and the Knapp Commission; the rise of the ''Compstat'' system for monitoring neighborhood crime; the shooting of Amadou Diallo; and the station house abuse of Abner Louima. His takes on these are interesting. He accepts, for example, the guilt of Justin Volpe, who was convicted of sodomizing Louima (''Thanks again, Volpe,'' he mutters when a crack dealer makes a scene on the street by yelling how the police sodomize people with broom handles). But in the case of Diallo, he talks about the serial rapist whom cops from the Street Crimes Unit were looking for that night; ''I wondered if Amadou Diallo died because the N.Y.P.D. thought too little of the South Bronx, or too much. Whenever I heard about the 41 shots, I thought about the 51 women raped.''

While sometimes the detached observer, Conlon is often deeply involved in the stories he tells, and to the extent that he's not self-aware, his tales can yield some unflattering insights. There is a faint echo of the Diallo case, for example, when his partner John Timpanaro pushes down a fleeing suspect, a drug addict. ''The perp had bounced off a wrought-iron fence before hitting the sidewalk'' and was still there when the ambulance arrived, ''unconscious and twitching.'' Timpanaro explains: ''He ran on me and I knocked him down, Eddie, and I swear, at first I thought I killed him. He ran down that hill, and I chased him, and he started to turn around with something in his hand.''

Something in his hand. Since Diallo, that phrase carries a resonance of police cover-up, which, oddly, Conlon doesn't seem to hear. He does, however, grill his partner, noting that ''John was a strong guy, the junkie was running downhill, and he could have flattened him with a flick of the finger.'' Then he quickly segues into the dangers of being too hard on cops who make an honest mistake. That turns out to be the moral of the story, the perils of politics intruding on the job. ''There's a wall at Police Headquarters full of names, and some are of cops who hesitated.''

More seemly, perhaps, might have been a little sympathy for the addict. Something similar happens when Conlon's beloved sergeant, P.K., in a patrol car speeding the wrong way down a one-way street, collides with a truck and hurts his head on the windshield. We learn how reassuring it feels to the author as the police shut down highways in order to speed their own to the hospital, but nothing is said about what happened to the truck driver. Informants, trying to collaborate their way out of drug charges, are encouraged to place themselves in harm's way throughout the narrative. Conlon portrays himself as more concerned about them than some of his colleagues are, but questions this impulse, and the trickle of payments that turn him into something like a junkie's employer (''Homeless junkies were part of the landscape, like pigeons, and the people who spent their days feeding them breadcrumbs had a hobby that was kindhearted but maybe a little unhealthy''). He can never quite bring himself to question whether some officially sanctioned aspects of police work are inherently dirty.

The department knows about Conlon's ''side job,'' as he refers to his writing career; top brass had to preapprove articles he began publishing in The New Yorker, as ''Cop Diary,'' by Marcus Laffey, starting in 1997. (Though he doesn't say, one assumes the book was vetted, as well.) Still, some higher-ups don't like the idea of his telling tales out of precinct, and, according to the author, his upward progress in the department is stymied more than once. (A friend says that his application to the Street Crimes Unit was shot down by supervisors worried about ''the prospect of bringing a reporter into the unit.'')

But these policemen had it wrong: he may love writing, but Conlon is no newshound, trolling for a juicy story or aspiring to be objective. Rather, the title is just right: Conlon bleeds policeman blue; ''the Job'' to him is a Catholic-style calling, a vocation. ''I liked the feel of a shield on my chest, and it began to make sense that you wore it over your heart.'' And the fact he's still on the job gives ''Blue Blood'' a charge and immediacy unlike any other police book I know.