Firewalls and Internet Security
Repelling the Wily Hacker
William R. Cheswick
Steven M. Bellovin
Published by Addison-Wesley
This book is a good introductory work to understanding network security fundamentals. It presents a good overview and technical foundation of how networking and various security protocols work, as well as providing case studies of security failures.
If you are more interested an crypto, you should look at Applied Cryptography, by Bruce Schneier. If you want a more historical view of crypto, look into Simon Singh's The Code Book.