Rowling's book was originally conceived as a limited edition
JK Rowling's The Tale of Beedle the Bard is on course to become the fastest-selling book of 2008 in the UK.
The Harry Potter author's collection of fairytales has shifted close to 370,000 copies in its first week on sale.
The book's nearest rival, Guinness World Records 2008, had 73,236 over-the-counter sales in the same period.
A spokesman for Nielsen BookScan, which compiles the official UK book chart, said sales were "huge", but had failed to match the Harry Potter books.
The seven-book saga has sold some 400 million copies around the world since the first instalment was published in 1997.
'Perfect Christmas treat'
Six handwritten copies of Beedle The Bard were originally produced as gifts to close Rowling's friends.
But a seventh copy raised almost £2m at auction, prompting the author to publish the 128-page book and give the proceeds to a children's charity.
JK Rowling made seven handcrafted versions of the book
More than seven million copies have been printed in 27 languages.
The book - which was first referred to in Harry Potter and the Deathly Hallows - contains five fairy tales, with "commentary" and footnotes by Potter character Professor Dumbledore.
Reviews have been positive, with the Times describing it as "a perfect Christmas treat ideal to slip in any stocking."
The Telegraph said the book "would barely satisfy Potter fanatics for more than half an hour" but said it contains "some nice touches".
Proceeds from its sale will go to Children's High Level Group (CHLG), a charity raising money for vulnerable children across Eastern Europe.
At a tea party held at the National Library of Scotland in Edinburgh recently, Rowling read from one of the seven handwritten copies of the book.