LIBRA; By Don DeLillo; Viking; $19.95.

By now, the assassination of John F. Kennedy - an event that novelist Don DeLillo calls "the seven seconds that broke the back of the American century" - has worked its way into the tribal lore of America.But we're still uncertain about the events leading up to the event. Why did Lee Harvey Oswald do it? Was he acting alone? If not, who put him up to it? Did the grassy knoll hold a second sniper? Any number of conspiracy theorists have applied themselves to those questions. Now it's the turn of a highly regarded novelist.

In an author's note, DeLillo mentions the chaos and clamor of conspiracy theories and half-apologizes for placing "one more gloom in a chronicle of unknowing."

Still, he says, "because this book makes no claim to literal truth, because it is only itself, apart and complete, readers may find refuge here - a way of thinking about the assassination without being constrained by half-facts or overwhelmed by possibilities, by the tide of speculation that widens with years."

Which seems to be a matter of having one's cake and eating it too, but more on that latter. First, DeLillo's way of thinking about the assassination.

Basically, it's a plot by rogue elements in the CIA, a plot that gets out of hand. The CIA renegades, embittered by the fiasco at the Bay of Pigs, decide that an assassination attempt (with emphasis on "attempt") will stir public wrath against Fidel Castro if the facts seem to lead from the Texas Schoolbook Depository back to Havana.

The plotters manufacture the facts as needed and paste them on the pliable form of Oswald. But they overestimate the reliability of the anti-Castro Cubans whom they recruit, and they overlook the fact that the Mafia has its own stake in seeing Castro run out - and its own leverage in the person of Jack Ruby.

Now, about having that cake.

Many of DeLillo's characters are real people - Oswald, of course; his wife, Marina; his mother; some of the Cuban exiles who float in and out of the real plot; even David Ferrie, the oddball pilot with CIA connections who serves in "Libra" as a Greek chorus of sorts.

Given the fact that so many of DeLillo's people are real, how does he expect readers to tuck his messy (and thus utterly believable) plot into a category marked "No claim to literal truth"? In fact, he paints such a grimly realistic picture of the Oswald family as life's little losers - often in the words of the Oswalds themselves, from Lee Harvey's Marxist ramblings to his mother's pitiful attempts at Southern-fried eloquence - that his story makes more sense than the Warren Commission's account.

But maybe it doesn't matter. Believe it or not, we're rapidly approaching the point at which a majority of Americans will have been born after Nov. 22, 1963. For most of us, the assassination will soon slip from memory into history, and history has always been fair game for an artist's imagination.

A novelist needs an imagination every bit as good as DeLillo's to keep us preoccupied with a story whose ending is known to everybody. So if Oswald's mind didn't run quite the way DeLillo says - well, what of it? Didn't Shakespeare take liberties with the mind of King Richard III? If nothing else, DeLillo has pinned down the underlife of Dallas in 1963, which is no small feat.

Still, as one who vividly recalls Nov. 22, 1963, I prefer my assassination fiction to be more clearly fictional. I'm still partial to "The Sisters," a wonderfully dark piece of assassination speculation written in 1986 by the gifted Robert Littel. But no assassination buff can pass up "Libra." It tends to suggest that the worst is all too mundane.