‘Whenever there’s a famous finish in the vicinity of a movie house, it behooves you to know what’s playing.’

‘I don’t doubt it, Mr. Banister.’

‘This is history with a f-ing flourish.’

So explains Guy Banister, the ex-FBI agent at the center of the conspiracy to kill John F. Kennedy, in Don DeLillo’s 1988 fictionalization of the assassination, “Libra.” What Mr. Banister calls flourish is the warp of fiction – the detail, the coincidence, the resonant context in the fact that “Manhattan Melodrama” was playing when the FBI caught notorious bank robber John Dillinger. The fictional tricks of Mr. DeLillo, however, go far beyond providing color. He actually suggests why things happen, at a level that history, these days, cannot honestly fathom.

“What makes a connection inevitable?” he asks. “It’s a line that cuts across causality, cuts across time. It has no history that we can recognize or understand. But it forces a connection. It puts a man on the path of his destiny.”

The postwar historical novel posits that only art can explain such connections. After all, histories and novels are only prose accounts of things that supposedly happened. The two forms would theoretically meet halfway. Readers crane to see Lee Harvey Oswald come alive on the page, turned inside-out so the reader can view the fictional contents of Oswald’s brain as he worries about his parents, or as he fires. Only Mr. DeLillo – no historian – can say that, as he was firing, Oswald was already talking to someone in his head, “telling how he was tricked into the plot.”

Two new books – Christopher Sorrentino’s, “Trance” (Farrar, Straus & Giroux, 528 pages, $26) and Mark Helprin’s “Freddy and Fredericka” (Penguin Press, 553 pages, $27.95) – try this trick with Patty Hearst and Prince Charles, respectively. But the special pleasures of this sort of historical fiction bring with them special dangers. The novelist, while storming the gates of history, puts himself and his horse in great peril. Such books make it impossible for a reader to suspend disbelief; instead, the reader will constantly compare fiction with fact.

Many authors display a nervousness about this. They pre-emptively link their characters with the historical record. John Updike begins his “Gertrude and Claudius” with a slurry of proper names that demonstrate his research into “Hamlet” and its sources while confounding the less-informed reader into trusting him. And Gertrude – or Gerutha, as Mr. Updike so correctly has it – winks at “the dark old days, when the deeds of the sagas were being wrought,” as if to remove Shakespeare’s drama and Mr. Updike’s romantic comedy to the same modern moment.

David Foster Wallace’s short story “Lyndon,” a portrait of President Johnson, combines narration with quotes from the historical record and contains a television-mediated description of Johnson on its first page: “The way he looked, when I looked at him, was always the same. He looked like eyes, the eyes of a small person, looking trapped from behind the lined hooked jutting face of a big bland bird of prey. His eyes are the same in pictures.” In these lines Mr. Wallace establishes his authority to write about Johnson by uniting his Johnson with the Johnson seen on television.

In “Trance” Mr. Sorrentino’s Tania Galton, an obvious cipher for Patty Hearst, eagerly consummates her television image as a willing, gun-toting terrorist. “Neither the crude disguise nor subsistence rations nor the rigors of combat training have altered a face everyone has come to know,” the author writes, his spate of “nors” typical of the automatic rhetoric in which this big, ambitious book floats. Mr. Sorrentino treats the believability of his historical fiction not as a problem but as an opportunity, reason to spin out a postmodern kaleidoscope.

Thus his epigraph claims that “Distilled to their essence, revolutions are acts of supreme creativity,” and the novel finds theater at every level, as if Patty Hearst were merely the epiphenomenon of a loopy epoch. A dishonest writer tempts the outlaws with a book deal, and in response Patty [Tania] produces a sardine tin, from which she withdraws three fish, to form the famous colophon of Mr. Sorrentino’s publisher, FSG.

Play like this drives “Trance.” The book is filled with writerliness – “It’s Nixon’s viscid gift that his presence haunts these hearings, clammily, despite his physical absence, his attempts to appear above the fray” – and throwaway characterization – “Haff called him the Shadow because of his ability to cloud men’s minds.” Yet Mr. Sorrentino seems most interested in book length conceits, layering narratives. He begins with a suspenseful novella, then inserts interludes, all the while fomenting confusion. Readers interested in Patty Hearst are advised to read Susan Choi’s 2003 novel “American Woman.”

***

Political power and heads of state suit historical fiction for a reason. One of Mr. DeLillo’s characters repeatedly observes that Kennedy literally glows “with secrets”; Mr. Sorrentino’s extremely centrifugal novel is held together by its constant interest in personal charisma. Mark Helprin’s wonderfully hilarious, endless “Freddy and Fredericka” – which would be more truthfully titled “Charles and Di” – asserts that secrecy is the essential element of royalty. In Mr. Helprin’s eagle-puffed prose, they live with secrets “as if on a battlefield of ghosts.”

The secrets Mr. Helprin seeks are not the barely adumbrated systemic trends of Mr. DeLillo, but inventions made out of wood and fairy dust. “Freddy and Fredericka,” pictures an alternate universe in which fumbling Freddy (Charles) is upbraided by Mr. Neil (Merlin), who once served Arthur (who is also Churchill), and who reforms bumbling heirs by sending them to conquer foreign lands. For Freddy, he has selected New Jersey:

“New Jersey is but a tile in a land so vast that, as far as anyone knows, it has no name,” Mr. Neil said in a mad whisper.

“Yes it does, you idiot,” Freddy told him. “It’s called the United States of America.”

“It is this, then, that you must conquer.”

So Freddy and Fredericka are dropped, naked (by ancient requirement), from a C-130, into New Jersey, where Freddy immediately employs his fencing skills to defeat a hoary motorcyclist. Later he speaks at the GOP’s presidential convention. So goes “Freddy and Fredericka,” for more than 550 pages. The plot ricochets like “Candide,” and, like “Candide,” its real satiric power lies not in the reversal of any particular maxim, but in the sheer accumulation of mischance, which undermines all seriousness.

Readers of Mr. Helprin must always be not only patient but patriotic. The royal secrets – hidden chambers maintained exclusively by a series of kings and royal consorts, for example – will flatter readers who can bliss out on glowing embers and single malt scotch. In his narrow register, Mr. Helprin is virtuosic. Inverting the logic of secrets, he proposes that “the eyes of the king” are archetypically “fixed on constellations in daylight,” and he happily summarizes his vertiginous closing vision as a “gale of sensation.”

Congenial readers may read “Freddy and Fredericka” while their children read the new Harry Potter to much the same effect. This is a fine adult’s children’s book.