The Casual Vacancy is virtually guaranteed to top the bestseller lists in Britain, the United States and beyond, with Rowling's name enough to attract millions of buyers and give publishers and book stores a much-needed boost.
Yet even that may feel something of an anti-climax for an author who has re-written the rules of publishing and generated the kind of media hype and public hysteria normally reserved for Hollywood royalty.
As each Harry Potter instalment hit the shelves, hundreds of thousands of fans dressed up as wizards and witches and queued outside book stores from Toronto to Tokyo, and the sales figures were staggering – 450-million copies and counting.
The 47-year-old Briton said she was looking forward to a more subdued book launch this time around.
"As much as is possible I wanted this to be a normal book publication," she told USA Today in one of a handful of newspaper interviews she gave to promote the novel.
"Some of the furore that surrounded a Harry Potter publication was fun. I always loved meeting readers. I always loved doing events where I got to speak to readers, but some of it, candidly, wasn't fun at all.
"The thing took on a life of its own. Some of it was just sheer insanity, and I couldn't control it. I couldn't stop it. I couldn't rein it in. Incredible as it is to look back on it, I'm never going to be chasing that again."
That "insanity" may explain why the run-up to the publication of her story about a small town in southwest England where class prejudices are laid bare has been so low-key.
The Casual Vacancy, published by Hachette Livre division Little, Brown, has attracted press coverage that most authors could only dream of, but Rowling plans only a few appearances, including one in London on publication day.
The Casual Vacancy has been described by the New Yorker magazine as a "rural comedy of manners that, having taken on state-of-the-nation social themes, builds into black melodrama."
Its starting point is the unexpected death of Barry Fairbrother, but it segues into a story about tensions between the well-off inhabitants and their poorer neighbours living on a nearby housing estate.
For Rowling, who was a single mother living on state benefits when she first started writing the Potter stories, the theme of poverty and class prejudice was particularly important.
"The poor are discussed as this homogeneous mash, like porridge," she told the Guardian newspaper. "The idea that they might be individuals, and be where they are for very different, diverse reasons, again seems to escape some people."
Since those hard times she has become what Forbes magazine called the world's "first billionaire author", both from book sales and her cut of the record-breaking, eight-part Harry Potter movie franchise.
As befits a literary A-lister, The Casual Vacancy has been kept under close wraps, and the Guardian stated, "Even the publishers have been forbidden to read it."
That was swiftly denied by Hachette, although a spokesperson added that only those "closely involved" with the book had read it and it had not been "widely circulated in-house".
Rowling said she was proud of her first foray into adult fiction and had enjoyed the "sheer freedom" of the last five years since she closed the Potter saga.
If readers and critics turn out not to like it, she said, "I can live with that."
Rowling has confirmed that her next book is most likely to be a novel for children which is almost complete. She is also contemplating a "director's cut" of two Potter novels because she had been in such a rush to finish them. – Reuters