When author Maria Hummel’s publicist called to tell her that her novel “Still Lives” would appear as a selection on a book club started by Reese Witherspoon, she couldn’t believe it. “I remember just staring in the mirror and thinking the person I’m looking at is not me, the person this is happening to is not me,” Hummel recalled.

The day that Witherspoon’s Hello Sunshine club posted the news on Instagram last summer, the relatively unknown novelist watched her Amazon sales ranking skyrocket to No. 11 and No. two for Audible audiobook sales. According to market research company the NPD Group, Hummel saw a 103% jump in sales over three months. And it was a big career boost. Time Magazine later chose “Still Lives” as a 2018 top new summer read. She soon started seeing her book make international waves.

“With the explosion of more readers, my book was going to really interesting places, you know, up in Alaska, and then it’s in Denmark, and then it’s by a pool and then it’s in the jungle, so that was kind of fun to see,” Hummel said.

There used to be only one book club that really mattered to authors: Oprah’s. A personal recommendation on her daytime talk show could lead to hundreds of thousands — or even a million or more — hardcover book sales. But as publishing faces a rapidly changing landscape, from the closing of brick-and-mortar stores to the increased importance of audiobooks, a wider array of book clubs have sprouted up, fronted by influential celebrities, athletes and TV programs targeted to moms. The book clubs are also a springboard to finding books to option for movies and TV projects.

The list of high-profile stars showcasing their bookish sides has blossomed since Winfrey’s start in 1996. Personalities like Witherspoon, NFL athlete Andrew Luck, Emma Roberts, Sarah Jessica Parker, Emma Watson and even Barack Obama now share their favorite page-turners via online platforms. According to Kristen McLean of NPD, the book business has seen a weak year in overall sales, but online celebrity book clubs like Hello Sunshine give the industry a boost. Witherspoon’s fiction picks have managed to carve out a “phenomenal” 2% of all fiction sales since June 2017.

“This idea of celebrity book clubs in the current era definitely goes back to Oprah’s first book club, but since the rise of social media, it’s really taken us to a different kind of environment,” McLean said, adding that these celebrity endorsements are crucial for new authors. “It’s just a very crowded information marketplace and there are a lot of books being published.”

At a time when original content is Hollywood’s holy grail, these book clubs provide a fertile source for new material. When Oprah used her platform to celebrate a book, the movie adaption would frequently follow — from Bernhard Schlink’s “The Reader” to Alice Hoffman’s “Here on Earth” and Cheryl Strayed’s “Wild” (which Witherspoon herself starred in and received an Oscar nomination for). Witherspoon has taken up Oprah’s brief and more, keeping that process in-house and scooping up some of the biggest bestsellers. Some of Hello Sunshine’s picks have been optioned by Witherspoon’s production company (which goes by the same name), such as a television adaptation of Celeste Ng’s novel “Little Fires Everywhere” for Hulu, an Amazon series of Taylor Jenkins Reid’s “Daisy Jones and the Six” and the movie adaptation of Gail Honeyman’s “Eleanor Oliphant is Completely Fine” with MGM.

“We don’t ever option books and sit on them,” said Sarah Harden, CEO of Hello Sunshine. “Because at the end of the day, there’s nothing that makes us happier than helping an author make more money from royalties and sell their books. We also bring that life onto the screen. When we decide to option something, we put our heads down and figure out how to bring that to life.”

But in other cases, just being considered a celebrity’s favorite book is reward enough. Last year, Obama shared a list of some of his favorite reads on Facebook, and one pick, “A House for Mr. Biswas,” by V.S. Naipaul, saw a 2,300% increase of sales in three months, according to NPD. Luck’s club targets younger “rookie” readers, promoting classic titles like “Charlie and the Chocolate Factory” by Roald Dahl and “Gulliver’s Travels” by Jonathan Swift. For “veteran” readers, he suggests titles like “The Righteous Mind: Why Good People Are Divided by Politics and Religion” by Jonathan Haidt and “Caramelo” by Sandra Cisneros. This summer, Jimmy Fallon reinstated his own “Tonight Show” book recommendation segment (started in 2018) asking viewers to vote for their summer pick. He said during a July show that he received 1 million votes in a week. The winner? Author Mary Beth Keane’s “Ask Again, Yes.”

Celebrity book clubs are also helping draw customers to local bookstores. Papercuts JP, a small 400-square-foot bookstore in Boston, witnessed a sizable jump in online orders and walk-ins since being featured on Emma Roberts’ Belletrist book club in July, according to store owner Kate Layte. Every month, Roberts and her partner Karah Preiss pick an independent bookstore and a book pick. July’s winner was the recent hot release “Three Women” by Lisa Taddeo.

In the first month of releasing a new hardcover book, Layte would typically sell just three or four copies. With “Three Women,” she said that the small shop has sold at least 20 to 30 copies within three weeks.

“We’ve definitely been having foot traffic come in and say, ‘Oh, this is exactly the book I’ve been looking for, I saw you guys online,’ and then they’ll stay and buy a few other books and just really build up that rapport,” Layte said. “It’s really exciting to see the online [activity] become physical interaction.”

Both Belletrist and Hello Sunshine focus on female writers and protagonists in their monthly picks. For Hello Sunshine CEO Sarah Harden, the “heart of a book club” hearkens back to groups of women meeting and connecting with friends through their love of storytelling. Other female-run clubs like Emma Watson’s Our Shared Shelf focus on feminist topics, featuring books like “Why Men Earn More: The Startling Truth Behind the Pay Gap — And What Women Can Do About It” by Warren Farrell and “Every Woman’s Guide to Saving the Planet” by Natalie Isaacs.

“The whole mission of [Hello Sunshine] is putting women at the center of the story who have agency, who are not the side character, they are driving the narrative,” Harden said. “All of our book picks have that in common. Our company is founded on the belief that storytelling can shift culture, and if we can shift culture, we can shift the way that women get to walk through the world.”

As for publishers, McLean said they are “totally psyched” about celebrity book clubs, because the difficult task of selling books in 2019 easier. “[Publishers] haven’t forgotten Borders [closing],” McLean said. “What’s not to love when one of these guys picks your book?”