I got the BBC download of this after the episodes that were based on it. The two-parter is much better than the book, and the book is less coherent than the episodes. Particularly in the sense that it is often hard to understand the characters’ motivations.
Part of this is caused by the fact that the book is part of a series — or at least, part of a larger story arc. But most of it is doubtless due to Cornell’s growth and improvement as a writer (possibly also he’s better at scripts than prose, but I don’t think so: he wrote a great short story in The Times last Christmas).
Still, it’s not a bad read, all things considered.