And Then There Were None – 100 million
This is the best-selling book by British mystery legend Agatha Christie, first printed in 1939. It details a series of murders on an island in which each death coincides with a line from an old nursery rhyme, with the seemingly impossible crime explained in Christie's ingenious epilogue.
The sales number is an estimate: Christie was once called the world's best-selling author by the Guinness Book of World Records, with 1 billion books sold worldwide. Since "And Then There Were None" is her best-selling book, 100 million copies sold is a good ballpark figure.