A caveat for anyone who in anticipation of the new David Fincher film about the Zodiac Killer plans to read either of the books on which it is based:
Zodiac, by Robert Graysmith, is utterly terrifying. I've read it at least four times since high school; once since living in Austin. Each time it unsettles me something fierce.
The sequel, Zodiac Unmasked: The Identity of America's Most Elusive Serial Killer Revealed, also by Robert Graysmith, was written a few years ago, following the death of someone the author believes was the Zodiac Killer (and who is referred to pseudonymously in the first book). This book is not very good. Also, it doesn't reveal the identity of the Zodiac Killer any more than its predecessor.