The story is told by starting at the scene of the crime and tracing one narrative thread backwards in time as another goes forwards. Which is the kind of device I really enjoy, but the book took a long time to grow on me. The first half was completely put-downable, but the second half is superb. It ought to be easy to get lost in the backwardly told story but it works really well, like you are uncovering this from the archives, racing back through time to find out how it began in order to work out how it ends.