When commercial publishers want to claim "fair use" privileges in quoting from various sources, one of the guidelines is the question of whether their usage will somehow affect the value of the medium the quote was extracted from.
As I understand it, the emphasis is on competition. If I write a book about peanuts and extract a couple juicy quotes from a book titled Peanuts, and my book sells more copies than Peanuts, then the author of Peanuts might have grounds to sue me.
But suppose I write a political book that is very critical of another political book. I extract a couple quotes to illustrate just how deceitful this other book is. I then go one step farther and advise readers "Do NOT buy this bum's book!"
Obviously, the author's going to be more inclined to sue me. But would they be able to sue me on the grounds that I hurt their business under these circumstances?
To be clear, my book would not be really similar and therefore presumably would not be considered an example of competition. However, it would be very critical of the other book, going so far as calling the author a propagandist and recommending that people not buy his books.