My first time trying to review a true crime book, let's see how this goes...
Zodiac is a 1986 book by Robert Graysmith, an artist and cartoonist who was a youngster working with the San Francisco Chronicle back in the 1960s when the whole Zodiac murder thing happened. He wasn't part of the Chronicle's investigative team. He got obsessed with the case, and this is the partial story of his investigation into what we know and what he thinks he knows.
Graysmith wasn't just reading reports, either; he got personally involved with interviewing suspects, victims, and law enforcement. He has a number of his own theories.