From the Press Association as reported on Google News:

Potter prequel sells out in one day

A book featuring an untitled Harry Potter prequel by JK Rowling became the fastest-selling collection of short stories when its print run virtually sold out in just one day.

What's Your Story?, is a collection of postcard-sized short stories by 13 authors including Doris Lessing, Nick Hornby and Sebastian Faulks.
But it is the 800-word prequel to the Harry Potter series that had fans rushing to the tills, with 10,000 copies reportedly sold since its release on Thursday.

In a tantalising introduction to the story, Ms Rowling says her tale is not so much a self-contained story as a slice of narrative "from the prequel I am NOT working on".

Set in the 1970s, the story revolves around Harry Potter's father James and Sirius Black in their teenage years tearing around on a magical motorbike.

The book was released by Waterstones to celebrate the National Year of Reading and also features three short stories written by members of the public who won a competition. The store said all profits from the sale of the collection will go to Dyslexia Action and English PEN.
The store limited purchases to just two per customer. Other authors who contributed to the book include Irvine Welsh, Michael Rosen, Tom Stoppard and Margaret Atwood.

A Waterstones spokesman said: "The limited edition What's Your Story? postcard collection has very nearly sold out on its first day of release, though customers should rest assured that there are still handfuls of copies left around the country.
"The magnitude of sales easily makes it the fastest selling short story collection since records began in 1994. There just isn't anything comparable."