Rowling has said the book involved the most planning of anything she has written; no small feat given the strict plotting of the Potter books.
Earlier this year, she told her Twitter followers: "My research into the subject matter of Career of Evil literally gave me nightmares, which has never happened before. It's a scary book."
The novel has already spent 24 days in the Amazon bestseller list, even before its release.
As with "Galbraith's" first two crime novels, it will star private eye Cormoran Strike, a war veteran who lost his leg in Afghanistan, and his sidekick Robin Ellacott.
Career of Evil opens with Robin receiving a package through the post containing the severed leg of an unknown woman.
The plot hinges on the hunt for who is responsible, with Strike chasing the many people he fears may have a potentially fatal grudge against him.
In researching the novel, Rowling is understood to have spoken to former servicemen about their experiences, as well as delving into the most disturbing of crimes.
Speaking at the Theakston's Old Peculier Crime Writing Festival, in Harrogate, North Yorks, last year, Rowling said she had planned the novel meticulously using "colour-coded spreadsheets". "The planning is insane," she said. "This is the best planned book I will ever have written."
The Cormoran Strike novels are due to be adapted into a television series by the BBC next year, and are expected to continue for at least seven instalments.
The first "Galbraith" novel, The Cuckoo's Calling, was published in April 2013 initially to little fanfare. Three months later, Rowling was unmasked as its writer, and the book leapt into the bestseller charts.
