The Roe of Paris Trout : BROTHERLY LOVE <i> By Pete Dexter (Random House: $22; 253 pp.) </i>

Tautly and often exquisitely written, Pete Dexter’s “Brotherly Love” is a novel partly dimmed by its darkness. That may seem a truism--what else would darkness do?--except that in a few great works, as in solar eclipses, darkness reveals the flaring corona around it. Iago illuminates “Othello,” and the Snopeses backlight Yoknapatawpha County.

Paris Trout, a human monster, did a similar kind of limning in Dexter’s masterpiece of the same name. Pure evil is invisible; to measure blight, you need crops; and in any blight, you register the resistance of some strains along with the ruin of others. What we saw in “Trout” was the effect of such evil on a variegated human landscape, with its shares of innocence, rascality, complicity, weakness and heart. The world--a small Southern town--was context.

In “Brotherly Love,” there are three full-grown Trouts and assorted henchman minnows, but relatively little context outside their own turbid waters. They are three kingpins of municipal corruption, labor racketeering and Mafia operations in Philadelphia. Standing for the world--the context, the crop--there is only a hesitant rebel in one of the gangster clans, and there is the upright operator of a boxing gym.

The book’s setting is the uneasy relationship, part-alliance and part-warfare, between the established political and labor-union mob, and newer forces represented by what the old Irish and Polish mobsters refer to as “the Italians.” The story, spanning a quarter-century, is told in terms of the moral and physical destruction of a boss family on the union side.

Its founder is Charley Flood, a chieftain who retains a notion or two of loyalty despite his racketeering. After tangling with Constantine, the aged and ruthless boss of his Mafia allies, he is betrayed by his brother, Phil, and killed.

Phil cements his own power by working with a young Mafia faction to liquidate Constantine. In turn, he is killed years later, and his even more depraved son Michael takes over. Phil’s killing may have been an act of revenge by some remnant of the Constantine faction; more likely, it was the work of his own Mafia partners, conceivably with the connivance of Michael.

That, dryly, is what happens, though it is not all that happens; the killings, of course, multiply. But what Dexter tells us is how these things feel, as, over time, a kind of rough outlawry degrades to absolute ruthlessness (Constantine), how this degrades to family betrayal (Phil), and how this degrades to sadistic degeneracy (Michael).

He tells it through the person of an innocent who, as the years go by, will surrender some of his innocence but not all of it, and who, at the end, will administer a jagged, partial retribution. He is Peter Flood, Charley’s orphaned son.

As in “Paris Trout,” “Brotherly Love” begins with a child’s dreamlike apprehension of horror. In the book’s stunning opening chapters, Peter witnesses the tragedy and violence that set off its vortex of evil.

Peter is taking care of his baby sister in the front yard to which he is restricted because of Charley’s numerous enemies. The baby is the light of Charley’s life, and for the solitary boy, caring for her is his only way of winning a spark of warmth from his father.

The neighbor’s car comes barreling along the icy street, skids into the yard, and hurls the baby into the air. Before the driver can get out, his dog bounds over and mauls the corpse. Stricken, the man carries the dead child into Peter’s house--Charley is not home yet--and goes to the telephone.

He is a police lieutenant on the take, distantly linked to Charley’s men and closely to Constantine’s. His emotion is not contrition but terror. Over the phone he tells his associates to come at once to “make sure he don’t do nothing premeditated.” Charley, that is. And then: “He’s not going to like this at all.” Charley, we think, but as the other men arrive, we realize he meant Constantine.

The men are all concerned, seemingly, for Charley’s loss. When he arrives and moves blindly to the door for revenge, they restrain him. The solicitous Phil is the foremost restrainer and soother. Bit by bit we realize, again, that it is not concern but fear. Constantine, Phil tells his brother, doesn’t want the policeman hurt.

In the next chapters we see Charley torn between his raging grief and Constantine’s power. At one point, the old man himself comes over, shrunken, solicitous and implacable. When Charley finally makes his break--there are moments of unaccustomed sweetness between him and his numbly watching son--and kills the policeman, it is Phil who then leads him off to a meeting that is a trap. And when Phil comes back to tell the boy his father is dead, Peter jumps from his bedroom window, injuring himself seriously.

The sheen, the tension and the anguish of this first part is extra-ordinary. The wary transactions of the lumbering hoodlums, in the no man’s land between Charley and Constantine, between loyalty and betrayal, are set out impercep-tibly and with striking effect. Such writing is hard to match, and in the remainder of the book Dexter doesn’t manage to.

Peter’s mother, a dim figure all along, has a mental breakdown. Phil takes Peter into his home and brings him up along with his own son, Michael. He tries to win his loyalty--at one point he takes him to a woman for his first sex--but never trusts him.

We follow the two boys as they grow up. Michael, fat and cowardly, turns into a vicious pervert whose power, first as his father’s lieutenant and later as his successor, makes him a true monster. Peter, aloof and plucky, finds refuge working out in the boxing gym run by Nick, a decent, tough man who is fatherly with the boy and resolutely contemptuous of his uncle and cousin.

It is never more than a partial refuge. Michael, who hates Peter, claims him by kinship nonetheless, and makes him his henchman. Peter, until his final bloody revolt, works for his cousin in a state of emotional disassociation. He practices a despairing kind of protest by jumping from building roofs at not quite suicidal heights.

Dexter’s account of Michael’s and Peter’s intertwined lives tends to straggle. The bombings and shootings, Michael’s lethal efforts to consolidate his power, and Peter’s half-escapes lack the superb narrative tension of the first part.

The problem, though, is not in the narratives but in the characters. Dexter makes the despicable Phil both oily--in a well-rendered bluff fashion--and execrable. Michael’s monstrosity--he brutalizes a young groupie when he finds himself impotent with her, and terrorizes a loyal follower who is unable to pay a debt to him--is horrifically rendered.

But Michael and Phil--and, earlier, Constantine--possess the elements of evil while lacking its sweep. Paris Trout’s horror attained a dimension of mystery; the viciousness of these three is spectacular but flat. And neither the hesitant innocence of Peter nor the rather overloaded nobility of Nick are enough, as counterforce, to lend it resonance.