According to an archived article on the BBC website, Agatha Christie's books were unputdownable because she cunningly employed words and phrases that invoked a chemical response in readers' brains, using a technique not unlike hypnotism. (Thanks to Charmaine Clancy for the original link.)
Clearly, we need to know these magical words so that we can slip them into our stories and have readers falling under our spell and clamouring for more. The BBC says
These phrases included "can you keep an eye on this", "more or less", "a day or two" and "something like that".I can feel my eyelids drooping already.