As tourists plan to flood London with people for the Olympics, what should they know? A.A. Gill: “London is a city of ghosts; you feel them here. Not just of people, but eras. The ghost of empire, or the blitz, the plague, the smoky ghost of the Great Fire that gave us Christopher Wren’s churches and ushered in the Georgian city. London can see the dead, and hugs them close. If New York is a wise guy, Paris a coquette, Rome a gigolo and Berlin a wicked uncle, then London is an old lady who mutters and has the second sight. She is slightly deaf, and doesn’t suffer fools gladly.”