If you're concerned about the security of personal information on your computer--or in your e-mail--get PGP using this book. Garfinkel's guide to PGP (Pretty Good Privacy) encryption software is a comprehensive guide to secure encryption for everyone and anyone. So much so that even Phil Zimmerman, who created PGP, said he learned new things from this book. But more than that, it takes you behind the scenes into the fascinating history and workings of the great intellectual adventure story of cryptography. This book is a fascinating read as well as a top-notch guide, and is needed now more than ever.
As I write this in 2016, this book is over twenty years old, and is definitely no longer relevant to current PGP usage. However, it's worth reading (IMHO) if you're interested in the history behind PGP. The GNU Privacy Guard (gnupg.org) has largely taken over the PGP market, but it all started with Phil Zimmerman back in 1991.