SOMETIME on Friday afternoon, an unmarked van will pull up at the loading dock at Ottakar's bookshop in Darlington. Three pallets will be unloaded, each holding a stack of boxes. Each box will be numbered, but otherwise unmarked. The boxes will be locked in the shop's storeroom, with strict orders that they remain untouched until one minute past midnight. This is Operation Hedwig - an exercise almost military in its secrecy and attention to detail.

The reason is not hard to fathom, for those boxes contain one of the most valuable treasures imaginable. Inside each one, are eight copies of the most eagerly-awaited book of the century: Harry Potter and the Order of the Phoenix.

About 13 million hardback copies of the fifth instalment in the Harry Potter series are being printed, the largest print run in history. Around 8.5 million of those will go to the United States, 2.5 million to Britain. Online retailer Amazon has already taken advance orders for 875,000 copies, and this is just for the English-language version.

So far it looks as though Harry Five will even outstrip Harry Four - Harry Potter and the Goblet of Fire - when it goes on sale at a minute past midnight. The Goblet of Fire became the fastest selling book in history three years ago, when around 372,000 hardback copies flew out of bookshops on its first weekend.

At 255,000 words and 768 pages, the Order of the Phoenix is around a third as long again as its predecessor, which itself was around twice the size of the first three in the series.

But its sheer bulk - the reason each of those boxes will hold just eight books instead of the normal 12 - will not prove a barrier to its success. Such is the demand, that when Harry's creator JK Rowling wakes up on Saturday morning, she will be £30m richer than when she went to sleep the previous night.

And the precautions to ensure the contents of the book do not leak out ahead of midnight are not misplaced. When the manuscript was delivered to the publishers, only two staff were allowed to be involved in the editing process. Illustrator Jason Cockroft was not allowed to read it before he designed the front cover. Such is the legion of Harry Potter websites, if the story was posted on the Internet it could cost Rowling and her publishers hundreds of thousands of pounds in lost royalties.

Earlier this month, a truck driver was convicted of stealing pages from the book. Donald Parfitt, who was ordered to do 180 hours community service, said he found three chapters of the novel in the car park of the printers where he worked. He tried to sell the extract to a newspaper for £25,000. Two other copies of the book were found in a field about a quarter of a mile from the printers - and were handed over to a newspaper before being returned to publisher's Bloomsbury.

And yesterday it emerged that police were hunting thieves who stole more than 7,500 copies of the book, with a retail value of around £130,000, from a trailer on Merseyside.

Of the 1,500 copies being delivered to Ottakar's in Darlington, around half have already been ordered by customers determined to be among the first to find out what happens to Harry.

'THERE is such a level of hype around the book and the number of reservations we're getting is astronomical - partly because they have been waiting so long," says Nathan Thompson, children's book seller. "Despite it being a children's book, there is a large proportion of adults who are buying it for themselves."

And Potter fans have had an unexpectedly long wait for the Order of the Phoenix to arrive. After initially delivering a book a year, Rowling took three years to complete Number Five, prompting fears she was suffering from writers' block. At one point, she is said to have considered breaking her arm to escape the pressure of living up to the expectation.

In fact, the delay may have had more to do with her marriage to anaesthetist Dr Neil Murray, on Boxing Day 2001, and the birth of their son, David, in March last year, than any difficulties involving pen and paper. Indeed, Rowling is said to have had the plots for all seven books in the series worked out before the first one was even published.

Partly to whet the appetite - and partly to offset the writers' block rumours - Bloomsbury revealed the opening lines of the book earlier this year, which run: "The hottest day of the summer so far was drawing to a close and a drowsy silence lay over the large, square houses on Privet Drive... The only person left outside was a teenage boy who was lying flat on his back in a flowerbed outside number four."

As the perceptive will have guessed, this fifth instalment finds Harry a teenager, ageing as the books progress. But as well as finding him older, the Order of the Phoenix is also said to be a much darker book even than the fourth, which, in turn, was darker than its three predecessors. In a rare revelation, Rowling said the fifth book would see Harry discovering why Lord Voldemort killed his parents.

And Rowling is enjoying the fruits of her labours. The story of how the single mother scribbled the first novel in a caf in Edinburgh, lingering over a single cup of tea, is well known, but those days are far behind her. Rowling has an estimated wealth of £280m, £30m more than the Queen and a far cry from the £70 a week benefits she existed on before. Instead of a mouse-ridden two-bedroom flat, she now has a £500,000 home in one of Edinburgh's smarter districts, a £4.5m home in Kensington, complete with underground pool, underfloor heating, and 24-hour security, and a £1m house on the banks of the River Tay in Perthshire.

But as the most successful author in history, with sales of more than 160 million books in 200 countries, it is hardly surprising. Film deals have also proved lucrative: Harry Potter and the Philosopher's Stone took nearly £650m at the box office worldwide, and the film version of the second, Harry Potter and the Chamber of Secrets, has taken £301m so far. Filming is now underway on the third, The Prisoner of Azkaban.

But for now all attention is on the fifth, with the hype building over several months and reaching unprecedented levels this week, including a £200,000 film premiere-style party in London.

OTTAKAR'S in Darlington will be opening at 7am on Saturday, and is expecting even bigger queues than those which greeted the arrival of the Goblet of Fire.

"I really don't think the excitement then was anything approaching what is happening now," says manager Diane O'Neill. "We will have just about every member of staff working - even at Christmas we don't have that many people in.

Nathan adds: "There might be some children who grew up reading Harry Potter who are teenagers now and don't want to be seen reading it, but they are getting darker and most people are still reading them. It is still the younger children who are most excited."

For thousands of young children, and quite a few adults, there will be only one place to be on Saturday. And don't even think of asking them if they're free over the weekend - they will have better things to do.

l The book to rival Harry Potter, Shadowmancer, by North Yorkshire vicar Graham Taylor, is also published on Saturday. For your chance to win a copy, see tomorrow's Northern Echo