Zodiac is described on the cover blurb as an 'eco-thriller', and for a change the blurb is close to being accurate. The book's main character is an ecological-crime detective, busily hunting down evidence of corporations illegally dumping hazardous waste and using publicity stunts and clogging up discharge pipes with cement as his main weapons against these companies. The book takes on a decided thriller aspect with the introduction of gene-tailored bacteria, designed to 'eat' contaminates, but there is a variety that generates them instead. How these bacteria are tracked down and controlled provides the main thrust for this book.
The plot is the main driver here, characterization outside of the protagonist is definitely skimpy, and in places the ecological warnings (though presented with apparent good scientific backing) become a little too strident, in places reminding me of Philip Wylie's The End of the Dream. Unlike some of his later books, his message is delivered almost directly, with little in the way of satire, irony, or his by-now patented brand of humor. The plot moves rapidly and logically, with enough potential hazard in the situation to easily quality as a 'thriller'. This makes for a quick read, but without his special zing that would make this book stand out.
Definitely an early effort, not in the class of his Snow Crash or The Diamond Age, still quite readable, but probably a must only for Stephenson hard-core fans.