I'm hoping to stimulate a little discussion here. I see pretty frequent suggestions on this site for various penetration testing and hacking books, but not for many other security books. I'm guessing that the members here read other security books too, so I ask:
What is your favorite non-hacking security book?
My favorite security book is Network Security: Private Communication in a Public World by Charlie Kaufman et. al. The book focuses on network security protocols and has very lucid explanations of the cryptography involved, how the protocols work and what their shortcomings are. It doesn't have a lot of practical advice for system admins, but it really helped me to understand Kerberos, IPSec, SSL, etc.