It's not just Pippa Middleton: how on earth did they get that huge book advance?

Big fees don't always mean big book sales, so why do publishers keep paying them? Tim Walker finds out

Pippa Middleton begins the introduction to Celebrate, her new collection of home entertaining tips, by saying that she is "by nature an optimist". Which ought to come in useful, given the book has only sold around 2,000 copies in its first week on the shelves, and was at No 180 in the Amazon bestseller list at the time of writing. This comes in spite of a 50 per cent discount on the cover price, and a considerable publicity campaign by Penguin, which paid a reported £400,000 advance to publish Pippa's poorly-received tome. So far, that's about £200 per copy sold.

As a celebrity publishing flop, Middleton is in illustrious company. Julian Assange was supposedly paid £500,000 for his memoirs, which sold just 644 copies in its first weekend. Arnold Schwarzenneger's stellar career and scandalous personal life could shift no more than 27,000 copies of his autobiography Total Recall in the month after its publication. He reportedly received a seven-figure advance from his US publishers. In 2006, Wayne Rooney agreed to write five books over 12 years for HarperCollins. His advance was £5m. The second book sold just 6,000 copies in six weeks.

If she never earns any further royalties on the book, Middleton can still comfort herself with that £400,000 advance. The picture is less pretty for Penguin. Depending on formats and discounts, a publisher that spends half a million pounds on a book might have to sell three times that many copies to justify its magnanimity. Editors at major publishing houses have the budget and power to make offers of up to six figures. But a six-figure advance such as Middleton's would likely be signed off by the head of the Penguin division responsible. Anything larger than that – Tom Wolfe's $7m (£4.4m) for his new novel, Back to Blood; Keith Richards' $7m for his memoir, Life; the estimated $3.5m advance for the forthcoming Not That Kind of Girl: Advice by Lena Dunham – would go straight to the very top.

"There's a lot of pressure on editors to buy celebrity titles for autumn to corner the Christmas market," Tom Tivnan, features editor of The Bookseller, explains. "You often see some panic buying of B- and C-list celebrity memoirs. A lot don't make their money back. " Many such books are hostages to fortune: a handful of England players earned payouts from publishers prior to the 2006 World Cup, but when the team failed to perform, so did the books. Still, says Tivnan, without big advances, celebrity books might never be written at all: "Celebrities are being asked to do something for well beneath their usual salary. If you offer to pay Wayne Rooney £250,000 for a book, that's his weekly take-home [pay]."

Not every big advance ends in a Christmas turkey. Richards, for instance, sold well over a million copies of Life worldwide. Dawn French was paid £1.5m for her autobiography Dear Fatty; According to Nielsen Bookscan, it's now the UK's 67th bestselling book of all time.

What an advance gets you in sales:

* Dawn French: 800,000 copies for a £1.5m advance: £1.87 per book

* Keith Richards: 1m copies for £4.4m: £4.40 per book

* Wayne Rooney: 6,000 copies (after two months) for a £1m: £166 per book

* Pippa Middleton: 2,000 copies (after a week) for a £400k: £200 per book