For the past week or so I have been haunted by scenes from Rebecca Stott's book ''Ghostwalk'': prismatic light reflections in a modern studio and darkened candlelit staircases from a distant past. I keep wondering if I have fully understood it - or whether, if I dip back into it, I will discover things which I missed on my initial reading, as if there might still be a piece or two missing from the jigsaw. It certainly exercises the mind on many different levels.