The blue nowhere - Jeffery Deaver

Someone is killing people in Sacramento Valley. Seemingly unrelated, the deaths are perpetrated by a murderer who knows everything there is to know about the victims - who can kill them because of the intimacy he seems to have with them. An intimacy which is created by his ability to track their every move through the virtual world, as soon as they switch on their computer.

Streetwise cop Frank Bishop is detailed to the case, allied unwillingly to a young hacker, Wyatt Gillette, who is sprung from prison to pit his brilliance against the criminal's. But no one knows who to trust in an environment where everything is suspect, and pressing the wrong letter on your keyboard may mean death.

This is the novel that will make you hesitate every time you click on the box that says 'Are you sure you want to send this over the Internet?'.