The embargo has been lifted, and the first reviews have flooded in from across the media.
Below, Digital Spy rounds up what has proved to be a very mixed response to Rowling's first book for grown-ups.
The Daily Express - Emma Lee-Potter
"The tale is a needle sharp and darkly comic expose of today's class-ridden society... The book isn't flawless. Her writing style is direct and uncomplicated rather than literary and she has an annoying habit of shoving slabs of explanatory detail into brackets.
"But from an author with a £620 million fortune from seven Potter books, it is clearly a labour of love... I like it too. Some readers will be shocked at Rowling's departure from wizardry and magic but The Casual Vacancy is a highly readable morality tale for our times."
The Daily Telegraph - Christopher Brookmyre
"One marvels at the skill with which Rowling weaves such vivid characters in and out of each other's lives, rendering them so complex and viscerally believable that one finds oneself caring for the worst of them.
"That leavening is there, but the novel's lesson is that it won't simply be gifted to us... we have to wade in to help in ways that may be messy, unsatisfying and barely effective, but without which we abandon hope. Ultimately, The Casual Vacancy is a book that understands there are no magic wands."
The Guardian - Theo Tait
"The Casual Vacancy is a solid, traditional and determinedly unadventurous English novel... The book seems doomed to be known as Mugglemarch.
"The Casual Vacancy is no masterpiece, but it's not bad at all: intelligent, workmanlike, and often funny. I could imagine it doing well without any association to the Rowling brand, perhaps creeping into the Richard and Judy Book Club, or being made into a three-part TV serial."
Metro - Claire Allfree
"This may sound like a big, bleak book, yet Rowling remains a writer to gobble down as easily as a supermarket ready meal. Mainly because, despite its weight, The Casual Vacancy is pretty insubstantial. It has a crude, slightly ersatz quality about it at times, as though it's drawn in thick felt tip pen.
"She's clearly set out to write an Important Book. A pity then that it should often feel so bland."
Evening Standard - David Sexton
"For, although this is billed as Rowling's first adult novel, a billing it certainly lives up to in terms of obscenity and sexual content, it still feels in its focus on perplexed, wounded adolescents in a world of selfish, uncomprehending parents, fundamentally Young Adult in its focus.
"The problem for Rowling's legions of fans will be that she has forgotten to include any basic likeability in her characters here or any real suspense as to what will happen - or deliberately chosen not to supply it, now she no longer needs to do anything other than what she wants. The book is quite punishing to read and the view of human nature it takes is more fundamentally lowering than that of the most cynical French aphorist."
The Mirror - Henry Sutton
"Lovers of Harry Potter are in for a bloody great shock. JK Rowling's first novel for adults is foul-mouthed, brutal and extremely bleak... The C-word is used as both a noun and a verb, while there are depictions of drug abuse, self-harming, domestic violence, and tragic, avoidable deaths.
"This is realism, red raw. Yet The Casual Vacancy is a complete joy to read which might seem odd given the nature of the material... Of course, Rowling didn't have to write this novel but she has done a rather brave thing - and pulled it off magnificently. "
The Scotsman - David Robinson
"Although she is no great stylist – her prose is seldom more than serviceable - her portrait of Pagford has coherence, integrity and, especially in the set-pieces where we see the characters en masse, a delicious interconnectedness.
"It is far grittier, bleaker (and, occasionally, funnier) than I had expected, and - the acid test - I suspect it would do well even if its author's name weren't JK Rowling."
New York Daily News - Sherryl Connelly
"The Casual Vacancy, which one bookseller breathlessly predicted would be the biggest novel of the year, isn't dreadful. It's just dull.
"Rowling's strength was never her prose. It was her ability to create unforgettable characters and weave stories that held us captive. The magic simply isn't there in The Casual Vacancy. Indeed, the spell has been broken."
The Daily Mail - Jan Moir
"Can The Casual Vacancy ever live up to the hype? On balance, I would have to say no. Not unless you want to have more than 500 pages of relentless socialist manifesto masquerading as literature crammed down your throat.
"JK Rowling likes to describe her new book as a comic tragedy, yet there are few laughs to pierce the blanket of gloom in this bleak and rather one-sided vision of life in modern England. In addition, at 503 pages, it is overlong, slow to start and bogged down with detail and a confusing rash of characters. But then, who would dare to edit the most successful author in the world?"
The Independent - Boyd Tonkin
"Slowed down by its fussy class geography and wheezing plot-motor, the novel builds into a vividly melodramatic climax with these kids at its heart. And after the convulsion comes a sentimental coda that, in tone and setting, whisks us right back to Hogwarts.
"Rowling's writing can be laborious in set-pieces but picks up magic with the adolescent characters."
The Daily Telegraph - Allison Pearson
"Sometimes funny, often startlingly well observed, and full of cruelty and despair... as for the ending, dear God, it is so howlingly bleak that it makes Thomas Hardy look like PG Wodehouse.
"At the end of The Casual Vacancy, there is no wand to wave, no spell to make the horror go away. It is pitiless... almost nobody in the story loves anybody else, and the author isn't much kinder."
The Huffington Post - Andrew Losowsky
"None of it is gratuitous. Focusing on these elements makes this mostly tame book sound on a par with Trainspotting or A Clockwork Orange. It's not.
"Instead, The Casual Vacancy is a generally well-written book whose central theme is responsibility for those less fortunate, all the time imbued with ever-present British themes of class and notions of propriety.
"Would this book be published if it weren't for the name on the cover? Almost certainly. Would anyone pay much attention to it, and its message? Probably not... The Casual Vacancy is a good, though not great, book about small-town, small-minded England. What else did you expect?"
The New York Times - Michiko Kakutani
"Unfortunately, the real-life world she has limned in these pages is so willfully banal, so depressingly clichéd that The Casual Vacancy is not only disappointing - it's dull.
"The characters in The Casual Vacancy feel so much less fully imagined than the ones in the Harry Potter epic... characters are drawn in brisk, broad strokes, and with little of the complex ambiguity that fueled the later Harry Potter instalments. In fact, there is a vacancy deep in the heart of this novel."
The Associated Press
"So look, here's the thing: This. Is. Not. A. Children's. Book. If you're looking for what made Harry Potter magical - Wizards! Spells! Flying Broomsticks! - you're not going to find it. If you're looking for what makes J.K. Rowling magical - emotion, heart - you will.
"What could have been an unreadable story becomes something else in Rowling's hands, thanks to her gift of being able to make her characters complex and really, just human."
Have you read The Casual Vacancy yet? What do you think about JK Rowling's new book? Let us know your thoughts in the comments box below