This CNN story from 2014 discusses the publication of Gary Stewart's book: The Most Dangerous Animal of All.
The video quotes the publisher, Harper Collins, as saying that the book will "Conclusively identify his father as the Zodiac Killer." A year later, I think we can say, conclusively, that it did no such thing.
I understand that Harper Collins is in the business of selling books, and that promotional materials are often far from objective. However, it seems to me that in circumstances such as these, large, well-known publishers should shy away from making controversial, absolute statements like saying a book will "conclusively" solve a case as notorious as that of the Zodiac, especially when any objective reader could point out that it does not deliver on the claim.
A second point of interest is a quote from the author, Gary Stewart, which reads: "I felt it was my responsibility to share the truths that I had learned in a way that would leave no doubt as to the identity to this killer..." Again, I think it's fair to say the book as fallen short of this goal. Instead, Gary Stewart has become yet another person who has convinced himself, but few others, that he knows the identity of the Zodiac Killer. If you'd like to read some of Gary's thoughts about reception of his book, there's a link to an interview in this article.
Interestingly, the above video has some footage from an interview with surviving Zodiac-victim Bryan Hartnell. The interview does not directly relate to Stewart's book; rather, it seems to have been added just to fill out the story.