There is a delicious irony at the heart of writing mystery fiction: the author must know exactly whodunnit, why, when, and with what blunt instrument — yet somehow contrive to feel the thrill of not knowing while the story unfolds. Get the balance wrong and the prose goes flat, like a detective who has already glanced at the back of the box before starting the puzzle. Get it right, and you find yourself genuinely surprised by your own characters, typing at speed because you simply cannot wait to see what they do next.