The Cuckoo's Egg by Clifford Stoll. This book taught me how important it is to keep a log of events. These logs come in very handy when the problem one trying to debug spawns multiple complex systems.
Seconded. This is also a great read on how to track down a deep problem, going through all of the steps required to figure out where it was coming from. That's on top of it being an entertaining view of early computer networking.
