The Cuckoo's Egg
A great book (and subsequent BBC TV series) telling the true story of Clifford Stoll, an astronomy professor at UCB's Lawrence Berkeley Lab. A 75-cent accounting error alerted him to the presence of an unauthorised user (a cracker) on his system. The cracker, code named "Hunter", was breaking into US computer systems and stealing sensitive military and security information. Hunter was part of a spy ring paid in cash and cocaine, and reporting to the KGB.
["The Cuckoo's Egg", Clifford Stoll, London: Bodley Head, 1990, ISBN 0-370-31433-6, ISBN 0-671-72688-9].