CNN  — 

It didn’t seem like an auspicious purchase at the time: a discarded library book bought for £1 (about $1.20) at a rummage sale, one of a handful picked up to read on vacation.

Except the book happened to be a first edition of “Harry Potter and the Philosopher’s Stone” (titled “Harry Potter and the Sorcerer’s Stone” in the US), one of only 500 to ever exist and distinguished by its two typos.

And on Tuesday, it sold for £28,500 ($34,500) at auction.

First editions of the title have commanded even higher prices at auction – a copy which belonged to J.K. Rowling’s first literary agent sold in London earlier this year for £68,812 (currently $83,350). Still, for a £1 investment, £28,500 is a pretty good return.

The misspelling of "philosopher" on the back cover is unique to the first print run.

Bought about 20 years ago, the book languished half-forgotten in a closet until its owner, who wants to remain anonymous, decided to raise money to fund a new bathroom. She invited Jim Spencer, a rare books expert at Hansons Auctioneers in Staffordshire, England, to value three boxes of books – one of which contained, in Spencer’s words, the “holy grail of Harry Potter books.”

Spencer is frequently inundated with calls and emails from people who believe they own a first edition Harry Potter book, according to Jill Gallone, head of media at Hansons Auctioneers. At one point, he was receiving “50 emails a day from people all over the world,” Gallone told CNN.

Few, however, are the real deal. Books from the first print run can be distinguished by the two typos: the word “philosopher” is misspelled on the back cover, while the phrase “1 wand” is repeated in an equipment list on page 53. Additionally, on the copyright page, Rowling is identified as “Joanne,” rather than the initials “J.K.”

“You get the book in your hand and you can immediately see these basic typos,” Gallone said.

The repeated phrase "1 wand" on page is another way to identify a first edition.

Books from the first print run are so rare, she explained, because “they nearly all went to schools and libraries.” By June 1997, when “Harry Potter and the Philosopher’s Stone” was first published, Rowling’s manuscript had already been rejected by a series of other publishers. “When the owner bought it all those years ago, nobody knew that Harry Potter would turn out to be this popular,” Gallone said.

Hansons Auctioneers expected the book to take between £20,000 and £30,000, but the owner had doubts. No one had placed an advanced bid online, so she assumed interest was low. That proved not to be the case: “There were three phone bidders that really battled it out to own the book,” Gallone said. Ultimately, it was won by a private collector in the UK, who also wants to remain anonymous.

The front cover of the rare book.

One reason for the book’s immense value? “It’s just simply that people love the Harry Potter stories all over the world. Rowling wrote a story that the whole world loved,” Gallone said. “This is a book from the very start of her journey.”

The buyer needn’t be a Potterhead to appreciate their purchase, however – as one of just 500, the book should be a sound financial investment. “It’s only likely to go up in value,” Gallone said.