As co-owner of a small business I read "Secrets of Computer Espionage" with interest, particularly since I thought that running a firewall and virus protection made my system fairly safe from intrusion. Hardly.
Consider the book required reading if you run any flavor of Microsoft Windows and need to keep private information private, and especially if your computers support external data connections (read: internet or external access).
McNamara covers a wide range of possible computer attacks from the mundane to the geeky-obscure. And he discusses the reasonable likelihood of each different type of attack along with how to identify and defend against them.
He also keeps the book common-sense, such as don't invest tens of thousands of dollars in attack-hardened hardware and software if you leave your building unlocked. He talks about physical access restrictions, company security policies, and other considerations regarding the ultimate goal of keeping the contents of your computer system safe - points that some computer security books overlook.
Finally, I love that McNamara injects humor, a conversational tone and many case studies into his book. It turns a potentially dry topic into an enjoyable read.