Daniel Radcliffe, Rupert Grint and Emma Watson in Harry Potter and the Order of the Phoenix. Photograph: Warner Bros./PA
The sales figures of the Harry Potter novels are like athletics achievements. At some point, you assume, they will reach the limit of what is physically possible. Harry Potter 4, The Goblet of Fire, broke all previous records for sales in a single day. HP5, The Order of the Phoenix, topped that performance. Then came volume 6, The Half-Blood Prince, setting a new bar at 2,009,574 copies in 24 hours.
A few hundred thousand of those copies arrived at people's homes from the Amazon warehouse. But most of the rest changed hands in bookshops and supermarkets. How many copies is it possible to transfer from one side of a till to another in a single day? The question will receive its most thorough test ever on July 21, publication day for Harry Potter and the Deathly Hallows, the seventh and final volume in J K Rowling's series.
The signs are that the record will once again be smashed. Amazon says that its preorders, standing at 2 million copies worldwide, are unprecedented. So too are Waterstone's. Tesco says that it expects to sell 350,000 copies of The Deathly Hallows in 12 hours - in 2005, the supermarket sold 400,000 copies of The Half-Blood Prince in 24 hours. Bloomsbury, JK Rowling's publisher, while advising that "we never release our print run", reports that export orders are up on last time; one can assume that UK orders are up too.
Harry Potter and the Deathly Hallows will sell in one day more than half as many copies as the total number of books sold in the entire UK book market in an average week. Its first-day figure will be more than double the one achieved by Peter Kay's The Sound of Laughter - the biggest selling hardback of last year - in the three months before Christmas 2006.
Surely the appearance of the fastest selling books of all time must be the best thing ever to happen to the book industry? Not entirely. The book industry has perverse features, and would find downsides to world peace, a cure for cancer, and a British victory at Wimbledon.
Some other children's authors have benefited. One can argue that Philip Pullman, deserving of all his accolades though he is, might not have achieved such a high profile without Rowling's achievements. Children's blockbusters by the likes of Eoin Colfer and Michelle Paver are post-HP phenomena. But the research company Book Marketing Ltd says that, "our data suggest that HP does not do any huge favours for the rest of the children's market".
But booksellers have mixed feelings about Harry. He was one of the factors contributing to the demise of Ottakar's, which incurred big marketing costs in promoting him before seeing supermarkets and Amazon cream off most of the sales. Waterstone's, which bought Ottakar's, has warned that it will not make any money out of The Deathly Hallows. Independent booksellers cannot compete with their loss-leading rivals; a quarter of them have said that they will not stock the book.
Even Bloomsbury has Harry headaches. The huge profits that the novels generate lead the City - in many ways, a stupid organism - to expect the company to make high margins in the Potter-less future. But publishing is publishing: a risky, low-margin business. Harry's wizardry can alter that law only for the books in which he stars.
