Let’s begin with the headlines already making their way to the gossip columns: Christina Haag reportedly earned a $1.2 million payday for her recollection of a five-year romance with John F. Kennedy Jr. In it, she reveals that he once took a few tokes of pot while on vacation in Jamaica. He also brought along a book on Tantric sex.

Big deal. We never even find out if the book’s lessons were put to use, though Haag portrays herself as a willing and eager participant.

The salacious bits do a disservice to the whole of Haag’s memoir, “Come to the Edge,” which, at its core, is a tale of aching, unresolved love. Perhaps even the best and steadiest loves are never fully resolved, but some — especially those that end because of mismatched timing — can haunt a life with the specter of empty, forsaken promise. Haag’s readers can only wonder, as she must, about what might’ve been had Kennedy felt ready for a permanent commitment when she did.

The two met first as New York City prep-school teens in the mid-1970s. The book gets off to a slow start here, with long descriptions of Haag’s Catholic elementary-school days and the history of her parents’ union, neither of which does much to inform our understanding of the narrator or her approach to romantic relationships, though it’s interesting to note that her parents weren’t born to the Manhattan wealth they eventually had.

For a couple of years, Haag and Kennedy were in overlapping cliques, hanging out on the Upper East Side, sneaking cigarettes and constantly searching for an apartment devoid of parents. Jacqueline Kennedy Onassis’s well-appointed penthouse on Fifth Avenue, it turned out, was a particularly good spot for trying to drop water balloons on pedestrians down below.

"Come to the Edge: A Memoir," by Christina Haag (Spiegel Grau. 275 pp. $25)

The book picks up once Kennedy and Haag both enroll at Brown University. For one year they lived in a group house, with their other good friend, Christiane Amanpour, whom they all called “Kissy.” Both dated other people throughout college; their most passionate encounter came in the form of a fight when Kennedy failed to fulfill his roommate duty to buy groceries for the week.

The Kennedy whom Haag knew was stubborn, charming, tender and presciently aware of his unique lot in life — with all the possibilities and limitations it implied. Like Haag, he was drawn to theater, but even the professor who was most appreciative of his talents knew it was not a path the son of JFK could seriously pursue.

Still, after college, when Haag finished her training at Juilliard and Kennedy was working for the City of New York, they began rehearsals for an Irish play directed by a friend from Brown. They’d gone to Jacqueline Onassis’s country estate in New Jersey to run through scenes when Kennedy first kissed Haag while they sat outside, admiring a blue moon. “I’ve been waiting to do that for a long time,” she recalls him telling her. But both were in serious relationships at the time. Haag and Kennedy’s romance was as messy at the beginning as it was at the end.

But by early 1986, both had extricated themselves from their other affairs and were dating exclusively. Kennedy introduced Haag to his family, including his mother, whose opinion clearly carried a profound weight with her son. Haag paints Onassis as welcoming and warm, a woman who reveled in the exploits of her children and their friends. Even after Haag and Kennedy broke up, Onassis looked out for Haag, sending handwritten notes and thoughtful gifts. At one point, Haag recalls, after an inept attempt to participate in a volleyball game, Onassis told her, “I was watching you earlier — you reminded me of me.”

Haag lyrically and precisely recaptures the frenetic ecstasy of early love. “I’m obsessed with you,” Kennedy told her. “You make me an emotional person, and I’m not.” She fell with equal abandon, and the requisite jealousy came as other women pushed phone numbers into his hand.

They were 20-something and searching — she for success as an actress, he for some kind of purpose. But even as he wafted halfheartedly between jobs and law school, his passion for adventure was a constant. On that trip to Jamaica, he prodded Haag, who’d recently broken her ankle, to kayak out to another island, though they’d forgotten their life jackets. “Couragio, Christina!” he bellowed. On the way back, they nearly hit a reef and were sucked under by a strong current that almost refused to let them go.

“We could’ve died,” Haag told him.

“Yeah, Chief,” he replied, “but what a way to go.”

Earlier Kennedy had told her he couldn’t imagine their love ever ending, but as the 1980s wound down, he wanted to see other people. He was happy, she recalls him saying, but needed time before settling down. They came together and apart several more times before saying a final goodbye. “No one will ever love you as I have,” Kennedy declared on his way out the door.

Haag never tells us if his prophesy came true. She leaves us hanging on other fronts, too. Did they keep in touch? Did she ever wed? What did she feel when she found out Kennedy had married Carolyn Bessette in 1996? And that he did it on Cumberland Island, Ga. — where he’d vacationed with Haag and first told her he loved her.

Regardless, it’s evident that Kennedy remained lodged inside Haag long after they parted ways. After receiving a cancer diagnosis, she sat on a bluff overlooking the Pacific, and it was him she felt with her, an arm draped across her shoulder. I hope Kennedy was wrong, and that Haag did find someone to love her as he did — just as much, and perhaps more resolutely.

McCarthy is a writer for The Post’s Style section.

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