Four billion sold — and counting. No, I’m not talking about Big Macs; it’s the number of Agatha Christie novels that have been sold around the world. Only the Bible and Shakespeare are more popular. Dame Agatha was born 120 years ago, but she and her most beloved creations, Miss Marple and Hercule Poirot, are still going strong. Consider the evidence:

PBS’s Masterpiece plans to bring both Miss Marple and Detective Poirot back for encore performances in 2013. A new book chock-full of unpublished letters, papers, and even part of an unpublished novel has just been published. Written by John Curran, it’s called Agatha Christie: Murder in the Making. A brand new edition of Agatha Christie: An Autobiography complete with a CD, has just been released. And to fuel the fires of her fans, more than 80 of Dame Agatha’s novels have recently been republished.

What’s the secret behind Agatha’s incredible staying power? Most likely it’s a blend of accessibility and consistency.

Readers enjoy entering the world that Agatha created and meeting the characters who inhabit it. They like the idea that this world is threatened, but that it is restored by the intervention of a quirky detective who outwits the forces of evil.

Then there is Agatha’s consistency: She was incredibly prolific and turned out scores of books over a long career. Readers turned into fans and began looking for her books; by writing them quickly, Dame Agatha kept herself top of mind and built an adoring audience that eagerly snapped up her newest creations.

Lessons learned? We may not be mystery writers, but we can certainly aspire to the combo that made Agatha a household name. We can strive to create appealing and accessible worlds, memorable characters, and lots of stories. Write on!