Every schoolchild recognises their protagonists: the Avengers, the X-Men, your friendly neighbourhood Spider-Man. The superhero comics that Marvel has published since 1961 make up the biggest self-contained work of fiction ever created: over half a million pages and counting. Eighteen of the 100 highest-grossing movies of all time are based on it. And not even the people telling the story have read the whole thing - nobody's supposed to.