Rowling was launching "The Tales of Beedle the Bard" on Thursday with a tea party for 200 school children at the National Library of Scotland in Edinburgh, where she lives.
The author is donating royalties from the book to a charity, which hopes it will raise millions to help vulnerable children.
Recession-hit booksellers hope the book - a collection of five fables mentioned in Rowling's saga about boy wizard Harry Potter - will give them a festive boost
"We expect it to come straight in at No. 1 and is very likely to be our No. 1 book this Christmas," said Jon Howells of Britain's Waterstone's book store chain. "It's in with a fighting chance of being the best-selling book of the year, even though there are only a few weeks to go.
"This is J.K. Rowling. None of the usual rules apply," he said.
"Beedle the Bard" is being published Thursday in more than 20 countries, with a global print run of almost 8 million. But is generating only a fraction of the fanfare that greeted the Potter novels.
Rowling is donating her royalties to the Children's High Level Group, a charity she co-founded to support institutionalized children in Eastern Europe. The book is published on behalf of the charity by Harry Potter's traditional publishers - Scholastic in North America and Bloomsbury elsewhere.
Rowling, whose Harry Potter books have sold more than 400 million copies and been translated into 67 languages, wrote the Beedle tales after finishing "Deathly Hallows" last year.
One of the stories, "The Tale Of The Three Brothers," is recounted in "Deathly Hallows," in which the storybook helps Harry and his friends defeat evil Lord Voldemort.
Rowling has described "The Tales of Beedle the Bard" as a distillation of the themes found in the Harry Potter books, calling it her goodbye to a world she lived in for 17 years.
The book was initially produced last year in an edition of seven handwritten copies. Six were given away by Rowling as gifts, and one was bought by Internet retailer Amazon at an auction for almost $3 million.
Amazon is printing 100,000 copies of a leather-bound collectors' edition priced at $100.
By Ben McConville