In the past, many successful children's writers have struggled to remake themselves as adult authors. Winnie the Pooh creator A.A. Milne, a successful playwright in his early years, once confessed that he was forced to say "goodbye to all that" after his beloved books about the bear and friends.
But Rowling has one advantage: the Potter books had a huge adult, as well as child, audience.
Jon Howells of British book store chain Waterstones said "The Casual Vacancy" would likely be the year's best-selling novel.
He said the synopsis came as a surprise and suggested similarities to the work of popular mystery writer Alexander McCall Smith and Mark Haddon, a children's writer who had a huge adult hit with "The Curious Incident of the Dog in the Night-Time."
"We were all expecting some sort of crime or mystery novel, but she has, refreshingly, surprised us all," Howells said.
"The plot sounds like it will have hints of Mark Haddon and McCall Smith, and the promise of black comedy is very beguiling."
Jill Lawless can be reached at: http://twitter.com/JillLawless