Down the Rabbit Hole by Lynne Jamneck

FBI Agent Samantha Skellar and her partner Robert Munroe are on the trail of a possible serial killer who has been terrorizing the suburban neighborhoods of Seattle.

When Agent Skellar starts receiving threatening messages on her computer, she calls in the help of free-spirited hacker Lucy Spoon, and gets much more than she bargained for.

Does someone have a personal grudge against Sam? Or could the elusive killer be targeting Sam for reasons unknown to her? And what about Lucy, who seems to be guarding secrets of her own...