"MY GOD, said the king, striking the youth knight, is the princess pregnant again?" So runs the ideal first line of the ideal popular novel, encompassing in brief the principal elements of best-selling fiction: religion, royalty, violence, sex and mystery. True Confessions offers no kings and princesses but it does have a Cardinal Archbishop and plenty of what passes in America for noble blood - hard cash. For the rest, John Gregory Dunne, the author of three previous books which just missed success, has neglected none of the essential ingredients, and the results is a tempting brew - a bit spicy for some tastes, perhaps, but substantial nonetheless. Substantial enough, at least, to be the Book-of-the-Month Club November selection.

Judged simply as a policier, the novel deserves high marks. A rather grisly sex murder gets the story under way; its investigation leads, with a certain grim inevitability, to the unravelling of the public and private lives of a dozen characters. Early on, you'll probably figure out who the villain has to be, but you'll be wrong or, better, half-right. Dunne plants the seeds of the deception in the opening chapter, set 28 years after the murder. Looking back, retired police Lieutenant Tom Spellacy reflects on how a clever remark of his turned a bizarre but apparently insignificant homicide into a continuing front-page story that shattered the facade of the city and archdiocese of Los Angeles and eliminated his brother, the Right Reverend Desmond Spellacy, as the leading candidate to replace the ailing cardinal. Only one man's fall in this Byzantinc world of graft and public service, of kickbacks and By JOHN B. BRESLIN; JOHN B. BRESLIN. S.J. is a corresponding editor of America magazine papal knighthoods, could effect such a catastrophe, and Dunne liberally sprinkles clues pointing in his direction. Ultimately, of course, another set of clues solves the case. But the obligatory twist serves more than the arbitrary ends of the genre. For what Dunne wants to explore are the ironies that shape people's lives, sometimes tragically but just as often providentially. The Catholicism here is more than cassock-deep, we should note.

The principal ironies affect the lives and fortunes of the Spellacy brothers -- "Me and Des. Des and I Des and me." as Tom sums it up at the beginning of the novel. Two harps from Boyle Heights who took the traditional roads out of the Irish ghetto: the church and the police force. Unencumbered and possessed of a nimbler brain, Des travelled faster and farther, becoming chancellor of the archdiocese in his thirties, a kind of ecclesiastical Haldeman, guardffice and firing elderly monsignors in His Eminence's stead.

For Tom the road to advancement deadended in Wilshire Vice when a peculiar set of circumstances, compounded onot a little black humor, maked him out as a bad risk. He barely escaped prosecution -- always wondering. did Des fix it? -- and landed in Robbery. Murder, over 40 and still a licutenant. But Tom's problems are more than professional. Married to a frigid. pious wife who has drifted off into a convenient dream world of mythical saints and parish gossip, he seeks solace else - where, only to panic when the women he sleeps with offer love as well.

Despite their separate paths. Des and Tom keep colliding.In bed with his wife. Tom is convinced she would much prefer Des. a thought little short of blasphemous to Mary Margaret Spellacy. Later, his mistress seeks out Msgr. Spellacy to confess her adultery. not because she wants absolution but to exorcise the phantom she senses between her and Tom. So much for the Freudian undercurrents. On the surface the sibling rivalry takes the time - honored form of civilized Celtic warfare: a battle of words and wits. Each survives in his separate jungle by a quick eye and a sharp tongue. Always keep one step ahead of your opponent. Never reveal anything without a clear advantage. In short, get the edge and hold onto it. When they meet, they simply switch into high gear:

Desmond Spellacy thought. It's always been like this.Push. Probe. Find the nerve.

"Tell me something. Des. What're you going to call your - self when you become Pope?"

"Simplicius II. that has a nice ring about it," Des said. "Or Gelasius III. There hasn't been a Gelasius since 1119."

"You been boning up."

"He who is prepared. Tommy, is neverzed lemon into his tea. "But then, on second thought. Gelasius has the ring of the Dark Ages. Something simpler. More to the common touch. "He picked up the teacup. "Thomas. After you. There's never been a Pope Thomas. Thomas the First."

"Thomas the First. I like that "Tom tapped the empty beer bottle. "Nice and common."

"I like it. too." Des said. "A constant reminder to me that the flesh is weak."

Tom Spellacy's smile hardened. He wondered if Des were fishing. Or if he knew about Corrine.

As the police investigatiion speeds up and the life of the murder victim unfolds, the brothers' banter strikes harsher and harsher notes. Des's friends and associates> the contractors, financiers and lawyers who are busy laundering their money and their reputations in the archdiocesup in Tom's files. Nobody is quite what he seems in postwar LA. And when Tom decides, in a moment of justifiable rage. to pull down the whole house of cards by arresting the kingpin, who. he knows. did not kill the girl, he is fully aware that Desmond will fall as well. What he doesn't know, couldn't know because Des barely understands it himself,is that he is doing his brother the greatest favor of his life. "You were my salvation, Tommy. "And so Dunne's fundamental irony proves to be providential rather than tragic. and double too, for Tom never fully appreciates what he has done, even at the end when the brothers are reconciled.

So much for the sage of the brothers Spellacy. For a supporting cast. Dunne has brought together a fascinating array of over-and underworld types. An enterprising detective captain who dresses in Sidney Greenstreet's castoff white suits and makes motel deals in Chinatown. A shrewd Cardinal who understands both high finance and low motives and can dismiss himself, in private, as a "bookkeeper in ermine." Ecclesiastical camp followers like Dan T.Campion, about whom Desmond muses: "If (he) had been at the Council of Trent, his only interest would have been who got the building contracts." And a strinf of pimps, whores, perverts and racketeers who daily confirm Tom Spellacy in his dark view of human nature. Turn the Spellacy boys loose on them, and the result is anovel full of marvelous comic scenes.

They make a grest pair. You may rightly prefer Simenon for your detectives and J.F.Powers for your priests, but Dunne has an adventage when it comes to mixed doubles.