She’s a beloved blonde pop star with the initials T.S. He’s a handsome N.F.L. player with a popular podcast and the initials T.K. Together, they have fused America’s obsession with pop stars and athletes into a celebrity pairing so incandescent it’s almost celestial.

That’s the story of Taylor Swift and Travis Kelce, the reigning prince and princess of American pop culture. But it’s also the plot of a recent work of erotic fan fiction, which some readers — many of whom are fans of Ms. Swift — are decrying as an invasion of privacy.

In a since-deleted TikTok video, which is still available on her Facebook page, an independent romance author who publishes under the name Ivy Smoak announced the release of a short e-book. In the video, Ms. Smoak claims to have written the novel in three days after receiving requests from readers for a “spicy” romance inspired by Ms. Swift and Mr. Kelce’s relationship. According to the trailer, the book would involve a “pop star princess,” a “football superstar,” a “sassy heroine” and a “secret relationship.”

On Sept. 29, less than a week after Ms. Swift’s first appearance at a Kansas City Chiefs game, Ms. Smoak published “Roughing the Princess” to Kindle’s Direct Publishing platform, which made it available for purchase on any Kindle device and as a free download for Kindle Unlimited subscribers.

The cover of the e-book features a cartoon image of a blonde pop star shown from behind. She is holding a microphone and wearing a red football jersey emblazoned with the number 69. While the book claims to be “inspired by” the real-life romance of Ms. Swift and Mr. Kelce, and starts out with a disclaimer stating that “any resemblance to actual persons, living or dead, events, or locales is purely coincidental,” many readers felt that the attempt at “shipping” — the act of imagining or championing a relationship between celebrities — hewed too close to reality, and they flocked to Twitter, TikTok and GoodReads to denounce it as inappropriate X-rated fan fiction.

In a review on Goodreads, a user with the screen name Isabella called the book, “almost creepy” in a 1-star review. “if i could give this 0 stars, i would,” she said. “i love shipping TS/TK but this makes it weird.”

To followers of BookTok — the nickname for TikTok’s avid reader community — there is a clear distinction between books that are loosely based on real people, and those that are “real person fiction,” a subgenre of fan fiction in which amateur authors create unauthorized fictional stories involving real people.

Many on BookTok felt that “Roughing the Princess,” which borrows exact details of Mr. Kelce and Ms. Swift’s biography and romance, while using nearly direct quotes from Mr. Kelce’s interviews and podcasts, read more like “real person fiction” than an “inspired by” story. That fact, coupled with the book’s graphic sex scenes, left many readers feeling uncomfortably voyeuristic, as if by simply reading the book they’d participated in violating their favorite pop star’s privacy. Devoted fans of Ms. Swift felt duped and disgusted.

“It’s fine to write a fanfic and it’s fine to write smut, but when you combine the two and then try to pass it off as an ‘inspired story’ when it’s very obviously about two real people, that’s not OK. That’s not right. That’s gross,” said Gina Burgess, 33, a TikTok creator who lives in Florida. Ms. Burgess made a video declaring “Roughing the Princess” to be “the worst book I’ve ever read in my entire life.”

“I read a lot of other books that have smut in them and I’ve never felt gross after them, but this one I did,” Ms. Burgess said.

Other “Roughing the Princess” readers echoed this sentiment in videos posted to TikTok and Twitter.

“This is not what the girlies wanted when we said we wanted a ‘football star meets pop star’ inspired romance book,” Kayla Compton, a social media manager for authors who has more than 54,000 followers on TikTok, said in a review. “It just felt like a gimmick to make some money.”

Ms. Compton, 26, said that as a football fan, a Swiftie and an avid reader, she felt compelled to finish the book, even though she wanted to stop reading after the first few pages. “I felt like I had to review it,” Ms. Compton said. “And I don’t ever do negative reviews, so the fact that I did it speaks to how much she shouldn’t have written it.”

Ms. Compton pointed to the book “The Playlist” by Morgan Elizabeth as a good example of a “spicy romance” that was inspired by Ms. Swift, but isn’t explicitly about her personal life.

Shortly after Ms. Smoak published “Roughing the Princess” online, it was yanked from Amazon, and almost all mentions of it were scrubbed from Ms. Smoak’s social platforms. It’s unclear whether Amazon unpublished the book, or if Ms. Smoak took it down herself; it’s also unclear if it was taken down as a response to reader feedback, or if there were potential legal or copyright reasons for the decision. (Ms. Smoak and representatives for Ms. Swift did not respond to requests for comment.)

Instead of wiping the book’s existence from her social media and continuing to post about unrelated subjects, some BookTok users expressed that they would have preferred for Ms. Smoak to have taken accountability for the release of “Roughing the Princess” and apologize.

“I think she could have come out and said, ‘Hey guys, I messed up,’” said Ms. Burgess. “I’m going to take time to examine what I should be writing and not take real events and make them into smut.”

This isn’t the first time Ms. Swift’s love life has been the topic of fan fiction. Her close friendship with the model Karlie Kloss has long been a source of “celesbian” fan fiction for so-called #Kaylor devotees who believe the two secretly fostered a romantic relationship. But many fans felt that Ms. Smoak’s book crossed a line by including graphic conversations and sex scenes involving two characters so clearly modeled in the image of Ms. Swift and Mr. Kelce, especially when Ms. Swift has been consistently vocal about how much she dislikes speculation about her love life.

“If you’re writing a Kaylor book, that’s not real life, you’re imagining things,” said Ms. Compton. She also pointed out that fan fiction is typically free, whereas copies of “Roughing the Princess” were on sale for $6.99. “Fan fiction either is written about fictional characters or, if it’s about real people, it still has a large element of fiction and it’s free,” Ms. Compton said. “When you write a spicy fanfic story and you’re paywalling it, that’s when it gets weird.”