I brought the topic to the dinner table and laid it down casually between the chicken and the salad. "Do you know that Jackie's going to be 50 on Saturday?"

"You're kidding.FIFTY? My gawd, she looks fantastic," said one friend as he took hold of a wing. "That's what $30 million dollars can do for you," said the second, who immediately stabbed her tomato in the heart.

The third and shewdest turned to me, waggling a fork in the air. "You're not going to write about Jackie's 50th birthday," he pleaded. "I can't bear to read another word about her; she is an absolute zero. There is absolutely nothing else to be said about her."

With that, the four of us fell upon Jackie, not ravenously, but with more appetite for the subject than for the chicken.

One chewed sympathetically on the details of her life with an alcoholic father, an unfaithful husband, single motherhood: Dallas. The second nibbled bitchily on tidbits about her clothes budget and her "culture," her leopard coat and unseemly wrangling over foreign wills: Money. The third fairly choked on his ire about the Jackie industry, listing the repulsiveness of Ron Gallela, "Jackie Oh!," the magazine covers: The Myth Merchants. But even in his conviction about his utter indifference to Jackie, he was absolutely passionate.

Jackie is 50 and holding. Holding our interest. A full 22 years after her marriage to Jack, 16 years after Dallas, there is surely nothing left to say about her, nothing "new," and she is still the best seller we've got.

Even the readers and friends who complain bitterly about the amount of space devoted to this woman make it clear in their letters and conversations that they have read every word.

Why?

I think it is because, fundamentally, Jackie is still a mystery. And we cannot put the book down. Our appetite is whetted by the sense that if we keep listing the clues - Mr. Green, the library, the knife - we will eventually solve the puzzle of her character. But all we get is a longer list of clues and nothing more nourishing than speculation.

It is odd, I know, to think of this most chronicled woman as a mystery. Still it seems to me that what keeps people hooked on Jackie is this: We know all the facts about her, but none of the reasons. We know what she has done, but not why. We know where she lives, what she wears, what she buys, who takes her where, but not, finally, who she is.

We have a mystery without a motive. And it leaves us, like people who feast on beansprouts, unsatisfied, foraging for more, unable to believe that this is all. The woman who has had her territory invaded as if she were Central Park has managed to maintain some private core. This core, in turn, feeds the appetite of the invaders.

It is odd but true that if she had gone on the tube and told all, like Betty Ford, she would be left in peace. If she had worked for this politician or signed her name to that petition or come out in favor of causes more grand than a landmark building, she might be able to walk down the streets of New York without seeing a camera.

But Jackie is like an inscrutable lover, and we are like those people hopelessly attracted by the flashing lure of the unknown.

We have all known the arm's-length man who won't share, the distant woman who won't let us get close. Accidentally or not, they keep us near by keeping us at a distance. Once hooked, we assume that they have "unplumbed depths" and strive to discover them. We will be the one, we tell ourselves, where all others have failed.

But after two years, perhaps we know one-tenth of the person, after five years one-fifteenth and, if we are very lucky, after 20 years we have learned that it isn't worth the effort.

I have no idea whether deep down Jackie is shallow or whether in private waters she is deep. I have no idea if she is more driven by money hunger or mother love. I haven't a notion what she thinks of Jack's meanderings, or Ari's body.

I don't even know if we already know all there is to know.

But after 50 years of her life and 20-odd years of ours, can it possibly be worth the effort?