Without a doubt, the Zodiac killer is one of the scariest characters I've ever come across in fact or fiction!
The book is basically the result of years of investigation by the author into the still unsolved Zodiac murders. Robert Graysmith was a cartoonist who worked at one of the papers that Zodiac continually wrote to during his killing spree. Graysmith poured thousands of his own hours over a number of years into the case, determined to solve a case flooded with clues and suspects, but seemingly destined to go unsolved.
Graysmith sets the scene of the times so well, and his attention to detail (which although sometimes borders on the excessive) helps the reader paint a picture of the characters involved (victimes, detectives and suspects). As a result, at times the book is quite disturbing (descriptions of some of the murders are pretty gruesome) but because of the fascinating story behind the whole Zodiac thing and the passionate way it's told here, it's difficult to put the book down.
I finished the book wondering how a crime where there were so many clues, so many opportunities to catch the person responsible, is still officially unsolved to this day (although there is a prime suspect who is mentioned in the book) and despite the gruesome and just weird nature of the crimes, it leaves you wanting to find out more.
The book has now been made into a major film 'Zodiac' and it's also very good and quite true to the book.