PUBLISHING IS LITTERED WITH the carcasses of pretenders to the Harry Potter throne, but this week the US publishing giant Random House put its money where its mouth is and announced that it was backing its choice for heir apparent, 21-year-old Christopher Paolini, with a $500,000 advertising campaign and a record one-million first print run for his second novel Eldest due on August 23.

Paolini has a strong claim: the 21-year-old wrote his debut, Eragon, the first book in his Inheritance Trilogy, at 16. His magical epic was self-published but soon sparked excitement on the internet. After a New York house snapped him up, Eragon sold more than two million copies in the US within two years and a further 190,000 in the UK, when it was published by Random in January.

There is more than an echo of Rowling’s story in Paolini: she fought to get published for a measly two-grand advance. But soon her career soared thanks to word-of-mouth recommendation. Random seeks the same for Paolini’s second novel, with a UK print run estimated at 200,000.

Bloomsbury is the master of word-of-mouth marketing, though the £1 million advertising campaign for Harry Potter and the Half Blood Prince, published today, will help. That is a gargantuan budget for an industry in which fewer than 5 per cent of books get any advertising at all, and then budgets are often only £10,000. Even big-hitters such as Patricia Cornwell and Wilbur Smith have advertising budgets only in the low six figures: Pan Macmillan backed Smith’s latest, The Triumph of the Sun, with £150,000 of advertising. Cornwell’s publisher TimeWarner is believed to have spent a similar amount to make sure Trace toppled Dan Brown from No 1.

Big budget advertising is rare in the UK because retailers swallow up the biggest chunk of budgets for “co-merchandising payments” of up to £40,000. These pay for space at the front of bookshops and supermarkets. Derek Johns, an agent, says: “If the chains say that they don’t want to support a book then all the money in the world spent on advertising will not make the difference.”

It is also why publishers in the UK are more reticent about revealing the figures for marketing and print runs than in America.They claim that it upsets other authors when individual spends are revealed. But the real prima donnas are the retailers, who do not like to think their rivals are getting a bigger backhander and are left with red faces when their support fails to pay off. And big-budget books do fail. Louis de Bernières’ Birds Without Wings turned turkey and The Harmony Silk Factory by Tash Aw, which Fourth Estate backed with what one insider called a “substantial fortune”, did not reach the Top Ten.

However, Harry Potter 6 is one book able to resist the rapacious demands of the big retailers. It will fly off shelves thanks to a new kind of marketing, termed “watercooler publishing”. The aim is for Harry Potter 6 to take the place of the iPod Shuffle as a must-have accessory: to become an event rather than a mere book.

Rival publishers are watching closely. As Patrick Janson-Smith, publishing director of Transworld, says: “With something as big as Harry Potter or Dan Brown’s next book you have to do everything that you can to make it work. You can’t leave anything to chance.”

Potter by numbers

Bloomsbury has spent an estimated £1 million to promote Harry Potter and the Half-Blood Prince. Here is where the money has gone:

632,000 Harry Potter and the Half-Blood Prince carrier bags

177,500 cover art banner posters

171,530 packets of Bertie Bott’s Every Flavour Beans (in independent retailers’ party packs)

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