In I'll Be Gone in the Dark, Michelle McNamara outlines in detail her thorough search for the Golden State Killer. While doing so, she weaves her research through a narrative of her own life and experiences. This effectively nestles a true crime/nonfiction book inside a memoir: she contrasts the timeline of the Golden State Killer's crimes against her own development as an independent investigator, flipping back and forth as the narrative allows.
By approaching the topic from such a wide variety of angles—the killer himself, the circumstances surrounding the initial investigation, the progression of forensics and law enforcement, the human experience associated with the case, the social environment in California during the crime spree, and her own experience as a researcher—McNamara casts a fairly wide and inclusive net when it comes to her intended audience.
In part, I'll Be Gone in the Dark is for people with a prior interest in the case at hand. The Golden State Killer has been a hot true crime genre topic for years, and McNamara would probably have been able to count on that audience regardless of her approach to her manuscript. Because she interweaves it with memoir, though, we can assume the audience to be a little wider than the typical true crime book: those who have been following the case, for one, but also those with a general interest in true crime, those with a specific interest in procedural law enforcement, those with an interest in the history of California, and those who just like a good story.