The Plague by Albert Camus is a superb existential book, but I found it more valuable a read of human drama and emotion than a how-to for stopping biological terrorism. Actually, The Plague also came into my mind after WTC II because it examines how people faced with tradgedy change their life perspective - i.e. they try assure loved ones how much they love them while they still can.
For a graphic nonfiction book (written like a novel) on viruses, nothing beats THE HOT ZONE by Preston about the Ebola virus.
For biological warfare, there was a frightening book out called BIO-HAZARD by one of the former heads of the Soviet Unions biological weapons program. The Soviet's had worked out many of the kinks for using biological weapons and unfortunately much of their workproduct can't be accounted for.
Thumbs up here too for THE HOT ZONE. A must read. This is not a work of fiction.