Celebrate? I doubt it… Pippa's party book tumbles down charts

After bagging a £400,000 advance for her literary debut, Pippa Middleton was determined to prove that she had much more to offer the world than a famous posterior.

However, Penguin may fail to recoup its investment in the Duchess of Cambridge's younger sister after her party-planning book failed to reach the bestseller lists.

Celebrate, the 29-year-old's collection of "personal tips, favourite recipes and useful decorating ideas", was published a fortnight ago, amid a wave of international publicity.

"It has been a crazy couple of years since my sister's wedding but it's had its upsides and downsides," Ms Middleton said at the book's London launch. "I just feel really fortunate to be able to build a career as a writer."

Mocked by critics for its banality, sales of the 416-page guide have been slow, with just 2,189 copies of the title sold during its opening week, making it only the 185th bestselling book of the week.

Sales picked up marginally in its second week, with Celebrate shifting 2,747 copies. But after entering the Amazon top 100, the book, discounted from £25 to £12, yesterday slumped to No 166 in the online retailer's chart. Across the Atlantic the story is much the same. Despite attending a publicity launch in New York, Celebrate was ranked No 266 in Amazon's list of US bestsellers. Penguin refused to disclose Celebrate's total sales figures.

The Middleton campaign has been hamstrung by the author's self-imposed silence. She has declined all interviews in order to avoid embarrassing her sister and the Duke of Cambridge – a policy which appeared wise following the international furore over the publication of topless pictures of the Duchess.

It has meant turning down US offers to appear on the influential Ellen DeGeneres chat show and a reported £500,000 NBC deal to present a new cookery show focusing on traditional British dishes.

Instead of answering questions, Ms Middleton wrote a newspaper article defending the book from charges that her party tips were dull and the recipes somewhat lacklustre. She wrote: "I could have written a book that showcased the most extravagant or elaborate of occasions – after all, I spent three years working for an events company planning parties – but I wanted to produce something that was achievable… Something that was not too fussy, just simple and comforting."

However, all is not lost for Celebrate. The book, racked prominently at WH Smith and Waterstones, is likely to catch the eye of Christmas-gift purchasers.

And Ms Middleton can also call herself a chart-topping author. Celebrate is No 1 in Amazon's list of bestsellers in "entertaining and special occasions", outperforming the Ottolenghi cookbook and offerings by Hugh Fearnley-Whittingstall, Delia Smith and Nigella Lawson.

And the international rights alone have delivered a return on a chunk of the Penguin advance, regardless of sales.

The Middleton family have been accused of using their royal connections to boost their business.

Party Pieces, the party-planning company founded by Carole and Michael Middleton, Pippa and Catherine's parents, is offering their daughter's book on its website for £19.99.