When Agatha Christie died in 1976, aged 85, she had become the world's most popular author. With sales of more than two billion copies worldwide, she had achieved the impossible - more than one book every year since the 1920s, every one a bestseller. So prolific was her output, it was even claimed that Agatha must have a photographic memory. Was this true? Or did she resort over those 55 years to more mundane methods of planning her ingenious crimes?