Nantucket, Mass., is a small island. And best-selling author Elin Hilderbrand makes it feel even smaller.

The 25-year Nantucket resident combs the tony town’s beaches, bars, restaurants — and even Facebook groups — for fodder that can be novelized in her bestselling beach reads (21 and counting).

“The specifics in the book, they come from daily life, they come from me really wanting to write about things happening in Nantucket,” said Hilderbrand, 49.

But it’s left some locals feeling a bit exposed.

“Nantucket is a much more discreet place than the Hamptons,” said one summer resident who has been coming to the island for 15 years. “It’s like, ugh. Elin is much more of a Hamptons character than a Nantucket person.”

There are even those who fearfully steer clear of the author. “Some people who are very openly swingers on Nantucket . . . stay off her radar and avoid socializing with her,” said the summer resident. “They don’t want to appear in the next best-seller.”

Since April, Hilderbrand has been posting a flurry of questions to a “Nantucket Days of Yore” Facebook group — in which locals and longtime visitors reminisce about the island’s history — to gather details for an upcoming novel, “Summer of ’69,” that she’s now writing.

Last month though, one of the members turned on her.

“One dude was like, ‘I feel like you’re getting our information for free and I feel used. You should just hire a research assistant.’ And I’m like, dude. Roll on past. If you don’t want to answer, don’t answer,” said Hilderbrand.

“He kind of took the air out of my balloon. I’m not here to piss anyone off.”

Which isn’t to say she doesn’t.

“On a [small, quiet] island like this, there are people who think too much publicity is not good,” said one Nantucket business owner of Hilderbrand’s books, which cover everything from torrid affairs to an islander running a prostitution ring. (Hilderbrand had heard rumors of a Russian one on the island.)

But not everyone is publicity shy.

Hilderbrand says that at charity events, she regularly auctions off the chance for people to be characters in her novels.

A friend of hers, real estate mogul Michael Lorber, joked that he’s “pissed she never uses my name in a book or uses me as a character.

“If John Grisham wrote about me ripping off real estate clients, I’d be very upset. But these are nice books about a beautiful time and place,” said Lorber who used to own a home on the island.

Last month, Hilderbrand even hawked a piece of jewelry integral to the plot in “The Perfect Couple,” her latest book, on Facebook. She posted, “For those of you who read ‘The Perfect Couple’: this is THE RING. I have one. I used it in the book. Say no more. If you want one, they are $495 with jewels or $250 without. Available from Jessica Hicks jewelry on Union Street here in Nantucket.”

Designer and store owner Hicks said she’s sold more than 10 rings since the book came out in June, including two custom 14-carat gold versions with diamonds and sapphires for $1,650 each.

“She’s mentioned my name in her books in the past, but this is the biggest uptick in business I’ve seen with orders and people coming in specifically to see the ring from the book,” said Hicks.

Residents interviewed by The Post say there are lines around the block whenever Hilderbrand sits for a book signing in town and the author says fans even sign up for the morning Forme Barre class she frequents just to catch a glimpse of her.

“Dudes arrange for me to meet their fiancées or wives as a surprise,” said Hilderbrand. “It happens every week.”

“I think it’s wonderful for me to be able to meet people while they are here,” she said. “It’s good business.”

The beach read queen is so popular that women from around the country trek to Nantucket in the dead of January for an Elin Hilderbrand fan weekend at The Nantucket Hotel.

“Women are just so invested in her novels,” said hotel owner Gwenn Snider, adding that Hilderbrand has penned many a novel on a yellow legal pad at the hotel pool while wearing a bikini.

Despite her brand, Hilderbrand didn’t grow up going to Nantucket. She was a Cape Cod girl, summering in Brewster, Mass., with her family as a child.

After studying creative writing at Johns Hopkins University, Hilderbrand moved to New York City to work in publishing, but quickly realized that a publishing gig didn’t make her a writer. She quit and taught 8th grade English for two years in Queens and Dobbs Ferry.

During her summers off, she headed to Nantucket to write. Her first summer there, she met the man she would marry, Chip Cunningham, who manages the Cliffside Beach Club (fictionalized in Hilderbrand’s first novel, “The Beach Club”).

In 1994, Hilderbrand moved to the island permanently, working a series of odd jobs as a substitute teacher, and writing classified ads for the newspaper. She and Cunningham married one year later. She took a two-year hiatus to attend the Iowa Writers’ Workshop where, cold and miserable, she started penning, “The Beach Club.” In 1999, Hilderbrand returned to Nantucket — and was toiling as a part-time paralegal when she sold her first novel.

Nantucket, she said, was an accidental muse.

“I’m not going to give quotes about inventing the ‘beach book,’ but I was certainly at the forefront of it,” said Hilderbrand, who has three children, ages 12 to 18, with her now-ex husband.

Her current boyfriend lives in Easton, Penn., which, unsurprisingly, is home to a few characters in “The Perfect Couple.”

When she switched publishers and went to Little Brown in 2006, they implored Hilderbrand to write about places beyond the tony island.

“But after [the 2008 novel] ‘A Summer Affair’ came out, they were like, ‘This is so successful, We’ve hit on something. Keep going.’”

She has. And the whole island has come along for the ride.

“After [2011’s] ‘Silver Girl,’ people started coming up to me and asking me to include them and their businesses in my books,” Hilderbrand said.

“I don’t do it for money. I should,” she said with a laugh. “But I don’t.”

Hilderbrand’s books are so dense with Nantucket minutiae, they can serve as a field guide to the island.

One resident said her friend was interested in dating a Nantucket vegetable farmer, John Bartlett, who owns Bartlett’s Farm — and another pal told her, ‘Oh, just read that Elin book about the island farmer. It was about him.’”

Whenever a new book by Hilderbrand is published, dinner party conversations on the chi-chi island — which is a part-time home for celebrities such as Google’s Eric Schmidt and actor Ben Stiller — commonly revolve around who inspired her latest coterie of fictionalized characters.

“I think there are definitely local people [who] are putting two and two together and going, ‘Okay, I know who this is about,’ ’’ said the business owner, who added that Hilderbrand recently declared she was going to steal a tale of his for her next tome.

“I was sitting with Elin just one week ago and I was telling some story about a hotel in Western Massachusetts that my wife and I stayed at and how the service went beyond, and she said, ‘That’s a great story, Let me use that in my next book.’ ”

While fantastic hotel service may seem like a dud of a storyline, Hilderbrand has a way of twisting real-life banalities into page-turners.

The writer recalled how, when her 2008 novel, “A Summer Affair,” came out, she ran into some awkwardness with a pal who was convinced the writer was dissing her in the book.

“I was co-chairing a benefit concert. And I had a co-chair, who is from New York, and I need to state, I absolutely love her,” said Hilderbrand. “In the book, I have a woman who is co-chairing with a woman from New York and she is just awful. So my darling friend wrote to me, ‘Is that me,’ she said? ‘Am I Isabel French?’

“And I said, ‘No, it’s not you.’ The novel couldn’t be about a wonderful co-chair. It had to be about someone who was truly abominable.”

But Hilderbrand admits, she’s not above exercising the power of the pen.

“When I was writing [the 2015 novel] ‘The Rumor,’ I was like, ‘I’m going to put every single person who gossips on this island into this book and this is going to be my revenge,’ ” said Hilderbrand, who laments that by the time the book was finished, the characters had changed so much that no one could even surmise the real-life inspirations.

“No one knew who was in there except for me,” she said. “Which was really satisfying, actually.”

Jessica Hicks Jewelry 

2 Union Street

Mentioned in: “The Perfect Couple,” 2018

A thumb ring from Hilderbrand’s favorite jewelry shop in town features prominently in this murder mystery. “People have been contacting me via Facebook and Instagram just to come see it,” said designer and shop owner Hicks.

Galley Beach

54 Jefferson Avenue

Mentioned in: “The Blue Bistro,” 2005

The novel, about a girl who works at a restaurant and falls in love with the owner, is set at a fictionalized version of the wildly popular Nantucket restaurant. “They’ve had various reactions over the years,” Hilderbrand joked of the real-life Galley Beach owners. “I think they were a little frustrated [dealing with the fans coming in and asking about her] — ‘Yes, we’re the Blue Bistro . . . Yes, we know Elin . . . Yes, she comes here.’”

The Club Car

1 Main Street

Mentioned in: “Here’s to Us,” 2016

“There’s this restaurant The Club Car, and attached is an actual car from the old Nantucket railroad where the bar is, with a piano. Every night people belt out songs. It’s one of my favorite things to do late night,” said Hilderbrand. “It’s had appearances in a few of my books.”

David Hostetler Gallery

42 Centre Street

Mentioned in: “Silver Girl,” 2011

Gallery owner Susan Hostetler was elated when her late husband’s art was mentioned in Hilderbrand’s novel. In the book, a Ruth Madoff-like character loses everything in the wake of her husband’s fraud, including a prized Hostetler sculpture. “I was very honored,” said Susan. “People were texting me and emailing, ‘David’s in the book!’ ”