Cecelia Ahern's second novel, Where Rainbows End, has made the top of the British bestsellers list, making it a double chart-topper.
The book by the Taoiseach's 23-year-old daughter has been at the top of the Irish bestsellers list since the novel was published at the beginning of November.
Now it has taken the number one position for original fiction in Britain. Last week alone, 17,000 copies of the book were sold in the UK, where it was first published in the middle of November.
WH Smith has ordered a special hardback edition for Christmas, which is unusual for a book by such a new author. The novel is being sold in large paperback format elsewhere.
In the British charts, the book has beaten The Graft by crime-writer Martina Cole to second place, followed by Night of Rain and Stars by Maeve Binchy, Going Postal by Terry Pratchett and Deep Black by Andy McNab.
The list appeared on The Bookseller website, which is widely used by the book trade.
In Ireland, 30,000 copies of the book have been sold so far.
The young author's first book, PS, I Love You, was a chart-topper in Ireland for most of the year in both paperback and hardback and remains in the top five in the paperback fiction list.
It has been published in 41 countries and Ahern is just back from Iceland, having completed a tour of Scandinavia. It also went to the top of the UK bestseller list.
"Cecelia is now well into her third book, which is expected to be published next year. She has proved she is not a one-hit wonder," Ms Marianne Gunn O'Connor, her agent, said.
She would not comment on any monetary aspects.
However, Ahern landed an Irish and UK contract for two books worth €300,000, a separate $1 million deal in the US, foreign language rights and a film rights deal worth up to €1 million for her debut novel.