The story is really well told, and the air of mystery is kept up right to the end. It's not a suspense, horror - there are no scary points in the book - its one where the mind follows the leads and gets absorbed in the full mystery of the case. The characters are endearing, I love the two old men by the end of it.
There's no character journey - none of them change - what there is a pure love of chasing the story. So there's no need for it.
It's a story that reminds me that what's important is the story telling, and that all of the theories are just that - theories.