When someone approaches you in real life because they know that your opinion is worth something, feel free to tell them whatever you want. That is not the issue here.
And in regard to this statement:
or advising that the content of a book might be offensive to some is not only l’toeles, it also meets the Chofetz Chaim’s requirement for purity of purpose,
‘cmon. I am sure you know very well that not liking the plot of the book or the use of the language isnot in any way shape or form what the Chofetz Chaim was referring to by offensive and purity of purpose. Please.