The Cuckoo's Egg by Clifford Stoll [1]
Pulling a small thread of trivial inconsistent accounting value to in the end exposing an international spy ring.
I grew up in the cold war, so I have always been interested in what went on. First recommended by a lecturer at Uni. The book reads like a techno-thriller but happened. Full of technical detail, I learnt a lot about UNIX system when I first read the book.
I would still say the book offers a great primer on computer security concepts and the logical processes people use to understand a systems. Always good to be reminded of that from time to time.
NOVA made an episode based on the book [2]