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“How do you write about a close friend?” Carly Simon muses in her new book, “Touched by the Sun: My Friendship with Jackie” (Farrar, Straus and Giroux), out Tuesday.

The question seems moot, since the relationship she describes seems less like intimacy than one-sided, abject adoration of a woman who didn’t always treat her kindly.

Jackie is Jacqueline Kennedy Onassis, style icon and former first lady. Plagued by paparazzi in later life, she zealously guarded her privacy.

It’s hard to believe she’d welcome this book, even though it comes 25 years after her death, of non-Hodgkin’s lymphoma, at 64.

Then again, Simon doesn’t give much away. Despite her own celebrity, the 74-year-old singer-musician is so blinded by hero worship that she can’t see straight. That, or she’s kept the juiciest stuff to herself.

Much of what we do see we pretty much knew or guessed already: that John F. Kennedy’s widow was effortlessly elegant in her “understated clothes,” and became Aristotle Onassis’ trophy wife because his money could help protect her and her children.

She was meticulous, polite and punctual, except when she wasn’t — which is how the “You’re So Vain” songwriter begins the book: not with their first, inconsequential encounter, in 1983 at a restaurant on Martha’s Vineyard, but several years later, on a snowy day in New York. They’re to meet for lunch at Café des Artistes, but while Simon arrives late, Jackie’s even later.

As she downs her wine and explores the bread basket, Simon’s heart races. She wonders if Jackie’s stood her up. What if she’d changed her mind about her, “decided I wasn’t worth knowing further?” Frantic, she runs to the bathroom, locks herself in a stall and pops a Valium.

Jackie arrives at last, having been stuck in an elevator. She laughs it off, while Simon dissolves with relief. Her neediness knows no bounds.

When Jackie asks Simon to find a band to play daughter Caroline Kennedy’s wedding, the musician is not only flattered, but grateful. She books the band and even sings with them. It never occurs to her how canny Jackie was to snag a Grammy- and Oscar-winning artist for free.

When you worship the sun, you often get burnt. Early in their relationship, Jackie, by then a book editor, contracts with Simon to do a children’s book, and asks the singer to name her price. She does, only to have Jackie tell her, years later, that the $25,000 she requested was chicken feed.

“Carly, you were screwed,” Jackie says. Never mind that it was she who did the screwing.

As Simon reminds us, the women — the daughters of a stockbroker (Onassis) and a founder of the publishing house Simon & Schuster (Simon) — had privileged childhoods. But money meant more to Mrs. Onassis. When Simon hooks up with a wildly attractive but unemployed writer, Jackie gives the relationship a thumbs down.

“Unless you let men be the ones who bring home the food, they will resent you for not letting them play their part,” she warns. Simon sticks with him, anyway. It isn’t until it’s clear that he’s gay that she ­divorces him.

At a dinner party with the Martha’s Vineyard elite, Simon interrupts the genial banter to ask Jackie where she got her beautiful sandals. The reply is terse, and it’s all Walter Cronkite can do to steer the conversation back to general ground.

Years later, Simon scours the city to find the only movie theater not showing Oliver Stone’s “JFK.” After meeting Jackie in the ladies’ room — where the former first lady waited in a stall for Simon’s all-clear whistle — the two settle in to watch “Bugsy.”

Just before the film starts, Simon asks her pal if she’ll ever see “JFK.”

“No, Carly, NO,” Jackie responds, and slumps into her seat.

That scene, raw and achingly real, is a rare moment in a book full of details that feel borrowed or apocryphal: the alligator scrotums used to upholster the bar stools of Onassis’ yacht; Jackie’s reaction to the fuss Simon was making over having the Clintons to dinner.

“Oh, Carly, for Christ’s sake,” the former first lady supposedly cried, “it’s just another president!”

After all is said and sung, Jackie remains elusive, a mystery wrapped in an enigma. And Simon knows it.

“My portrait will still be only a fraction, a trace . . . another ounce of blue added to the rose quartz,” she writes. “A painter’s palette gone off its rocker from trying too hard.”

Please, Carly, stop trying so hard. We haven’t got time for the pain.