Colleen Hoover was thrilled when her novel It Ends With Us suddenly shot to the top of The New York Times bestseller list late last year – and also a little bewildered. The book had come out four years before.

"You know at first, I didn't really know what was going on and even my publishers were like, why are we seeing an uptick in sales?" she recalls.

Hoover soon discovered that young readers were talking about her book on TiktTok, using the hashtag #BookTok, and Hoover had become a BookTok sensation.

People are buying a lot more books these days, in part because the pandemic has trapped everyone indoors with little to do. But among younger readers, another factor is driving sales up: The social media site TikTok, where users post short videos they create themselves.

It's hard to quantify how big BookTok is, because TikTok doesn't release a lot of analytics. But publishers say it has become a major force, especially in the market for young adult and contemporary romance books.

Scroll through BookTok and you'll find countless videos of readers talking about their favorite reads.

BookTok fans are mostly young women and they are ardent readers.

They prefer books that are passionate and emotional — and they like a good cry.

Not surprisingly, book publishers are more than a little excited about BookTok.

Word of mouth has always had a big effect on what books people read, and BookTok can turbocharge that process.

"It just warms my heart as a book publisher. Because that's what we always strive for, is that word of mouth. And we know that that's what takes a book from one level to the stratosphere," says Libby McGuire, who heads Simon and Schuster's Atria division.

BookTok comes along at a time when the pandemic has closed off many of the traditional ways of reaching readers, such as book signings, says Nellie Kurtzman, a vice president at HarperCollins, which published another surprise TikTok bestseller, They Both Die At The End, by Adam Silvera.

"There were a lot of book festivals where fans would gather and just celebrate reading and now not being able to do that, at least they're able to do that virtually," Kurtzman notes.

Publishers have even started reaching out to the most influential BookTok video creators, sending them free books or even paying them to recommend titles.

Chloe Gong, whose debut novel, These Violent Delights, was a bestseller last year, has a big TikTok following and often produces videos about her work and her life.

But she doubts these videos increase her sales much.

"A lot of the reason why book sales are moving so fast is not because of the authors themselves. It's because the readers are so passionate," Gong says.

McGuire agrees. The magic that turns books into hits on BookTok is something that can really only happen from the ground up, she says.

"I don't think you can manufacture it, I think it's happening organically, and that's great."

What is #BookTok reading?

We Were Liars by E. Lockhart

A tale of secrets and lies in a super-wealthy family that summers on an island off Martha's Vineyard. A best-seller when first published in 2014, it went on to Goodreads Choice Award for Best Young Adult Fiction and is a perennial BookTok favorite.

The Seven Husbands of Evelyn Hugo by Taylor Jenkins Reid

An aging movie star (think Elizabeth Taylor or Ava Gardner) recounts her career, scandals and all, for a magazine writer. Published in 2017, it's spent much of 2021 on the New York Times bestseller list, thanks to BookTok.

They Both Die At The End by Adam Silvera

Two strangers are told they're going to die before the day is out and decide to spend their last few hours together. Perhaps the quintessential BookTok success story. Published in 2017, it saw a sudden surge in sales in 2020 and has spent much of this year on the bestsellers list.

These Violent Delights By Chloe Gong

Gong was barely out of college when she struck gold with her debut novel. It's a Romeo-and-Juliet tale set in 1920's Shanghai during a gang war, and it became an instant New York Times bestseller. It's also helped turn the author into a BookTok favorite.

The Song of Achilles by Madeline Miller

An imaginative retelling of The Iliad from the point of view of one of its minor characters, it won the Women's Prize for Fiction (formerly known as the Orange Prize for Fiction). Thanks to BookTok, it's also spent much of the year on the bestsellers list, which is not bad for a book that came out a decade ago.

It Ends With Us by Colleen Hoover

A young woman whose childhood was blighted by domestic abuse struggles to stop the cycle of violence in her own life. Originally published in 2016, it suddenly shot back onto the New York Times bestseller list last year, where it remains. This book and Hoover's later novel Verity have made her a bonafide BookTok sensation.

The Love Hypothesis By Ali Hazelwood

Love and romance in a university laboratory. It came out just this past fall, and was an instant bestseller, as well as a BookTok hit.