"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.