The formula to a bestselling book is a mystery publishers have wrestled with for centuries. Now they may finally have their answer: encourage Taylor Swift fans to believe she may have written it.

An untitled book, known only as “4C Untitled Flatiron Nonfiction Summer 2023” by a yet-to-be announced author has shot up sales charts in the US, after “Swifties” pieced together apparent clues and came to the conclusion that it could possibly be a memoir by the 33-year-old singer, who is known for peppering her songs with easter eggs that reward devout fans.

The mystery book from Flatiron, an imprint of publisher Macmillan, is available for pre-order for US$45 (A$66, £35) and has been hovering in the top 10 of the Amazon book sales chart, rising as high as No. 2. It is also currently No. 1 on Barnes & Noble’s pre-orders chart.

The pre-order rush began when a document purported to be from Flatiron began circulating on social media, which said that the title and author will be revealed on 13 June. The book is also said to contain 544 pages; and while that would make for a hefty memoir, Swift’s fans have noted that 5 + 4 + 4 = 13. Thirteen is – common knowledge among Swifties – the singer’s lucky number.

The mystery book was described in the document as a “biography or autobiography” that would feature 40 colour photos. It will have an initial print-run of one million copies and a global release date of 9 July: the huge number and short lead time suggest it will be a famous author.

The 9 July release date also matches the date mentioned in the lyrics of Swift’s song Last Kiss, a track on her third album Speak Now. Swift is set to re-release that album on 7 July, as part of her “Taylor’s Version” project to rerecord the six albums she made for the Big Machine label headed over by music executive Scooter Braun, a man whom Swift despises.

Fans are also speculating that Swift’s decision to use “dear reader” in her announcement for the release of Speak Now (Taylor’s Version) was a clue that a book would follow.

“This is not a political book, it is a fun, celebratory title and will skew slightly younger, but is for people of all ages. This has global appeal and will have massive publicity. I would comp (compare) this to Flatiron’s Matthew Perry memoir … and a little bit to Spare by Prince Harry,” the document read.

Regardless of the scant detail, the hashtag #TaylorSwiftBook on TikTok now has more than one million views, as fans share their theories and excitement over a book they know nothing about.

Booksellers are also in the dark; Wisconsin bookshop Blue House Books wrote that they’d give full refunds “if it turns out not to be the memoir we think”. Another bookshop in New York shared on TikTok that it would be canceling more than 600 preorders for the book after receiving information that it was not by Swift. “It has been a fun and interesting (albeit stressful) experience getting embraced into the world of Swift,” they wrote.

On Tuesday, Variety said it “can report for certain that this mystery author is not, as rumoured, Taylor Swift”, adding that it was unlikely anyway given she is in the middle of a huge US stadium tour and about to release another album – “an excessive exercise in synergy even for someone as capable of project-multitasking as a Taylor Swift”, Variety’s Chris Willman wrote.

Speculation remains rife, but the firm favourite to replace Swift as the author is BTS, with several figures in publishing reportedly confirming off-record that the book is by the K-pop group, who are currently on hiatus as various members complete their mandatory military service in South Korea. BTS is expected to reform sometime around 2025 – so a 544-page book would tide the A.R.M.Y. over until then.

Flatiron has yet to comment on the speculation.