Barack Obama is taking time out from a busy schedule running the United States to write a foreword for Nelson Mandela's private diaries, due out in October.

The news last year that the former South African president's private collection of journals, letters, speeches and notebooks, covering his imprisonment on Robben Island, the anti-apartheid struggle and his time as president, were to be compiled into a book sparked a frenzied bidding war at the Frankfurt book fair. The announcement that Obama will be contributing a foreword is likely to send hype around the title higher still.

"We always had a hope that Obama might do it, and a plan that we would put a call in," said Jonny Geller, literary agent for the book at Curtis Brown. "Now it's been delivered and we realised it's better even than what I'd sold -- the quality exceeds the hype. It's brilliant."

The Nelson Mandela Foundation "opened the conversation" with Obama's office, said Geller, and the US president agreed to write a foreword to the book, Conversations With Myself, which will be published around the world this October.

"It feels right," said Geller. "The most significant event of the last few years has been Obama becoming president, and he'll be commenting on the most significant black leader in history."

Mandela's autobiography Long Walk to Freedom, published in 1995, has sold six million copies around the world. Obama is no slouch in the sales department himself, reporting income of $5,5-million on his tax return last year, most of it from his two bestselling books, Dreams From My Father and The Audacity of Hope. Combined, the pair are likely to have a huge impact on this year's bestseller charts. - guardian.co.uk