Null Pointer by Johnny Batch (PDF)
The man in the cubicle beside him was dead. The police say it was a heart attack, but programmer Joshua Jones knows better. A message he found in the dead man's code points to a psychotic hacker who can strike through the internet, leaving no trace. Relying on the advice from his tech savvy friends, Joshua must solve the mystery of who the killer is and how he attacks before Joshua becomes his next victim.