Taylor Swift fans made a mysterious book a best-seller. Then they found out what it really is.

(CNN) — An unpublished, as-yet-untitled book by an unnamed author shot up the U.S. bestseller lists this week, thanks to preorders from Taylor Swift fans who believed it to be her memoir.

But sadly for Swifties, the book previously listed as “4C Untitled Flatiron Nonfiction Summer 2023” has now been named: “Beyond the Story: 10-Year Record of BTS,” written by the musical juggernaut themselves, K-pop superstars BTS, according to updated listings on some booksellers’ websites.

BTS have written the memoir with journalist Myeongseok Kang. Flatiron Books, an imprint of Macmillan, will publish it on July 9.

So what convinced Swift’s fans to fork out $45 for a mysterious 544-page hardback?

It all comes down to numerology. First, the number of pages. When the individual digits in the number 544 are added together (5 + 4 + 4), the total is 13. In an appearance on “The Tonight Show with Jay Leno, “the “Shake It Off” singer confided that 13 was her lucky number.

Earlier in the week, a document allegedly from Flatiron made its way across social media, indicating the book’s author would be revealed on June 13, which Swifties took to be a second clue.

Next, the July 9 publication date is just two days after the release of Swift’s new album, “Speak Now (Taylor’s version).”

In a recent Instagram post announcing the album, Swift highlighted the date, which keen-eyed fans interpreted as a reference to the book.

“It fills me with such pride and joy to announce that my version of Speak Now will be out July 7 (just in time for July 9th, iykyk ????),” Swift wrote cryptically.

But both June 13 and July 9 are also of significance to the BTS community.

The septet debuted their first single, “No More Dream,” on June 12, 2013, and the book’s July 9 release date similarly marks a 10-year anniversary, “ARMY Day,” celebrating the founding of the BTS fan group known as the ARMY.