NEW YORK >> The guessing game began a few days ago, when an anonymous, untitled book scheduled for July 9 — “4C Untitled Flatiron Nonfiction Summer 2023” — had Taylor Swift fans so convinced she had written a memoir that they made it a bestseller.

By Thursday, the Swift rumors had been refuted and the mystery resolved: Flatiron Books announced the upcoming release of a book by another musical powerhouse, BTS.

“Beyond the Story: 10-Year Record of BTS” is 544 pages, according to the publisher, written by the journalist Myeongseok Kang and structured like an oral history about the K-Pop boy band. It has been translated into English by Anton Hur, in collaboration with Clare Richards and Slin Jung.

Details about the book had encouraged fans of Swift and BTS. The author and subject were to be revealed June 13, a favorite number of Swift’s, but also the 10th anniversary of BTS’ first single, “No More Dream.” The page count numbers — 5-4-4 — also add up to 13. The publication date, July 9, is close to the release date of Swift’s album “Speak Now (Taylor’s Version),” but it’s also ARMY day, the 10th anniversary of the founding of BTS’ ARMY fan base.

Speculation about the book’s subject made it a bestseller on Amazon.com and Barnes and Noble’s website.

Booksellers had been informed of a major release from Flatiron, which billed the work as a “fun, celebratory title” that would skew to “slightly younger” audiences. The book, a Flatiron sales official emailed sellers, would have “global appeal” and “massive publicity.”