To the undoubted joy of millions of fans — not to mention the world’s booksellers — J.K. Rowling announced on her website yesterday that she had completed the manuscript of her sixth blockbusting Potter adventure ahead of schedule.

It will not take the staff of Hogwarts Academy to work out why. Rowling is expecting her third child, and a manuscript romised to her publishers by Christmas Day has been delivered a week early.

The relief in the book trade was almost palpable. Bloomsbury, her publishers, gave due warning to the world that Harry Potter And The Half Blood Prince will go on sale in Britain and America on July 16 next year.

For those few non-aficionados of Potter, a half blood prince is a wizard with a “muggle”, or non-wizard, as a parent.

Rowling, who has made an estimated £435 million from her books and their spin-offs, has caused palpitations in the past among her readers and those who make money out of her. Her last work, Harry Potter And The Order Of The Phoenix, was originally expected in 2002, but did not appear until June 2003.

The author denied writer’s block on that occasion and proved her point by producing a 255,000-word manuscript. She admitted to a drying-up of her creative gland while writing Volume 2, The Chamber of Secrets, and to have been on the verge of abandoning the whole project while on Volume 4, The Goblet Of Fire.

In an interview with Jeremy Paxman last year, Rowling said that writing The Goblet Of Fire had not caused her serious trouble; it was, she said, “quite a docile book to write.” She did admit to five difficult weeks during The Chamber Of Secrets when the words would not come. She even contemplated breaking her arm to give herself more time.

Rowling, 39, who lives in Edinburgh, started writing Book Six the day she finished writing Book Five. She intends it to be the penultimate volume in a series which has created a phenomenon previously unknown to the publishing world. Phoenix sold 1.8 million copies on its first day in Britain and even greater volumes are expected this time.

Scott Pack, buying manager at Waterstone’s, which sells one in every four books bought in Britain, said: “There’s no comparison — no book, no video, no CD, except possibly BandAid — to the sale of a new Harry Potter. Each book so far has been the fastest-selling book ever.”

Addressing the legions of the faithful on her website to spread the good news of completion, Rowling said: “I know you all expected this to happen on Christmas Day, but I was sure that those of you who celebrate Christmas have better things to do on the day itself than fight your way into my study, as those of you who don’t celebrate Christmas would definitely prefer not to wait until the 25th.”

Whatever you think of Rowling as a writer, you have to admit that she’s got the hang of marketing.

Rowling has posted a teasing extract, which reveals nothing, on her website: “He looked rather like an old lion. There were streaks of grey in his tawny hair and his bushy eyebrows; he had yellowish eyes behind a pair of wire-rimmed spectacles and a certain rangy, loping grace, even though he walked with a slight limp.”

There is, however, no mystery about Bloomsbury shares, which leapt by 8 per cent yesterday to add £16 million to the publisher’s market value. There was clear relief in the City that, in the delivery stakes, book has come before baby.

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