When Alan Greenspan signed an $8.5m (£5m) book deal with Penguin last month, the former chairman of the Federal Reserve joined a select band of celebrities in the business book world who command multi-million-dollar advances.

They include legendary investor Warren Buffett - who is co-operating with Alice Schroeder on a "biography of ideas", for which Random House reportedly paid $7m last October - and Jack Welch, the former head of General Electric, whose second book, Winning, earned an advance of $4m.