Book excerpt: Cameron and Lisa Murdoch are successful crime-writers. They have been on the promotional circuit, joking that no-one knows how to get away with crime like they do. After all, they write about it for a living.So when their 7 year old son Zach goes missing, naturally the police and the public wonder if they have finally decided to prove what they have been saying all this time - are they trying to show how they can commit the perfect crime?