The fifth Harry Potter book is to be published with both children's and adults' covers on launch day, for the first time.
In 1990, when JK Rowling started writing a children's book about a schoolboy wizard, she could hardly have predicted how Harry Potter would go on to enchant adults as much as her younger fans.
After Harry Potter and the Philosopher's Stone was released in 1997, commuters were spotted covertly reading the brightly coloured book behind their newspapers, prompting Bloomsbury to issue a paperback adult edition in 1998.
This year the publishers are bringing out the hardback adult version of Harry Potter and The Order of the Phoenix at the same time as the children's edition in response to consumer demand.
"Families were going into bookshops and buying up multiple copies on the first day of sale. Adults could not wait for the paperback grown-up version to come out," said a Bloomsbury spokesperson.
The adult cover for book five
But what do adults see in the bespectacled boy and his magical misadventures?
Lewis Carroll and Roald Dahl have always fascinated readers of all ages, but Harry Potter is the first modern "crossover" book on such a huge scale.
All four published books made their way into the UK's Top 100 books, in a recent BBC nationwide survey.
Bloomsbury spokeswoman Lucy Chapman thinks that adults can find another level in the Potter books.
"People can read into the mythology that runs through the novels, they pick up on more, such as the Latin school mottos."
Joel Rickett, news editor of the Bookseller website, believes it is less complex.
That whole retro thing became hip in the late 90s and it was acceptable to regress back into childhood
Joel Rickett, The Bookseller
"It is about regression and nostalgia, it mimics the style of earlier children's writers such as Enid Blyton which is comforting to the older generation."
The main reason though, is because it is a brilliant read, he said.
"It is not great literature but it is well put together and is a page-turning light read."
Mr Rickett also cites the aura surrounding the author as a factor in Harry Potter's appeal to an adult audience.
The first adult cover was markedly different to the children's version...
"The books are the product of one person which gives them an air of authenticity. The romantic image of a single mother writing stories in a café is appealing."
He believes the Harry Potter phenomenon also coincided with a time when it became fashionable for adults to indulge in children's activities, such as computer games.
"That whole retro thing became hip in the late 90s and it was acceptable to regress back into childhood," he says.
And children's literature is also being taken more seriously than it used to be.
Philip Pullman's The Amber Spyglass, the third in the His Dark Materials trilogy, became the first children's book to win the prestigious Whitbread Prize in 2002, after Rowling had missed out earlier.
'Story the key'
After the presentation, Pullman told reporters the tale itself was crucial.
"The point about it is that it's inclusive. It doesn't shut anyone out. It doesn't say this story's only for girls or only for boys or only for women or anyone else.
...but plenty of adults read the children's versions anyway
"The story's the important thing."
But adult Potter books still have some way to go before they catch up with the children's editions.
At Borders bookshop on London's Oxford Street, the children's paperback version of Harry Potter and the Goblet of Fire outsells the adult version 10 times over.
"We don't really plug the adult versions in shop displays as the covers are a bit dark and dreary," says bookseller Gary Reynolds.
But whatever one's opinion of the cover, Bloomsbury will be hoping fans buy up both versions on 21 June.
Harry Potter and the Order of the Phoenix is out on Saturday.