The main point of Gillian Flynn's introduction to I'll Be Gone in the Dark seems to center on Flynn’s personal response to Michelle McNamara and her book, as well as Flynn's professional evaluation of McNamara’s talents.
The introduction begins on an autobiographical note. Flynn writes that McNamara, who died two years before the publication of her book, grew up in Illinois and knew that she wanted to be a writer from a young age. In the introduction, Flynn uses McNamara’s first name, which reinforces the personal tone of the introduction. Flynn even suggests that if they had grown up together, they could have been friends.
As the introduction progresses, Flynn switches to more professional considerations. She praises McNamara’s hard work and persistence. Flynn commends McNamara’s ability to organize and keep track of lots of details.
In the final paragraph, Flynn’s personal and professional points tie together. She says she wanted to know more about McNamara personally because of the strength of I'll Be Gone in the Dark.
Another way to think about the point of the introduction is as a personal endorsement. Flynn uses the introduction to make the case that not only is McNamara’s book on her relationship to the Golden State Killer compelling but McNamara herself is intriguing as well.
If Flynn were not a best-selling author of crime novels like Gone Girl, perhaps this personal endorsement would carry less significance. But considering Flynn’s relative renown, her praise might lead a fair number of people to read McNamara’s work.