Kept closely under wraps until publication day, the book is a gritty tale involving sex and drug addiction that is widely expected to be Britain's best-selling fiction title this year. It is already topping the Amazon charts.
Set in the fictional village of Pagford in southwest England, it tells the story of the fight to fill a slot on the parish council after the incumbent's sudden death, and hinges on the fate of a squalid housing estate.
It took Rowling, 47, five years to complete and required another half-written children's novel to be put on the back burner, she told her audience.
But writing it was "a lovely place to be -- there was so much pressure for the Potter books".
"I kept telling myself 'you don't have to publish this,'" Rowling said.
She spent Thursday watching "Men in Black 3" with her children in an effort to avoid reviews, she said -- and the book has met with a mixed reception from critics.
Several found it dull in parts despite scenes of sex and drugs, and that Rowling's most vivid writing was on the familiar ground of children pitted against the power of adults.
Famously tough critic Michiko Kakutani wrote in the New York Times that "there is no magic in this book -- in terms of wizarding or in terms of narrative sorcery.
"Instead, this novel for adults is filled with a variety of people like Harry's aunt and uncle, Petunia and Vernon Dursley: self-absorbed, small-minded, snobbish and judgmental folks, whose stories neither engage nor transport us."
But in Britain, Allison Pearson wrote in the Daily Telegraph that "The Casual Vacancy" was "sometimes funny, often startlingly well observed, and full of cruelty and despair".
And Boyd Tonkin in the Independent called it a "song of freedom" as Rowling was able to abandon the constraints of children's writing following her seven books about the boy wizard.
Rowling revealed Thursday that her favourite characters in her own books were Fats -- a foul-mouthed schoolboy in the new novel -- and Dumbledore, the bearded wizard, wise but troubled, who is headmaster of Hogwarts in the Harry books.
She said that she would change "quite a few things" about the Potter novels, and that the actors who played Harry and his magical friends Ron and Hermione in the blockbuster films were "all too good-looking".
Rowling has previously said she left "the door ajar" for a return to the world of Harry Potter, although she said she was not intending to write any more books about the young wizard.
Asked about the constant spectre of death in the new novel, Rowling described herself as "death obsessed".
"Why does it obsess me? I don't know. The easy answer is my mother died when I was 25 -- she was only 45. That was clearly a formative experience."
Writing about death had made her "less afraid of it", she said, but she was still "frightened of leaving my children".
Danielle Salvatore, 19, a student from North Carolina, had paid £200 ($325, 250 euros) for a resold ticket to the event.
"It's worth it because JK Rowling inspired a lifetime of magic for me," she said.
Rowling told fans she "hated" the intense secrecy which has surrounded each of her books before publication day, but said the risk of online leaks had forced her publisher's hand.
In advice for budding writers, she said they should "get an agent!" and not always expect to feel inspired.
"Inspiration is clearly necessary, but then comes the long and hard work of writing," she said. "I definitely don't wake up and ask myself, 'Am I inspired today?'"
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