Should we criticise others' books?
Well, if we want to keep our reputation intact, no. There are a horde of people out there waiting to jump on you. I used to be a lot more outspoken than I am now, but I couldn't take it any more.
You criticise a book, some authors take it as though you've criticised their firstborn child. There is, of course, criticism and criticism. Sometimes it's obvious that a critic is just being mean. There's no real comments about the book, sometimes it's about the genre, sometimes about the author, and the comments deteriorate into generalisations. Bad. And sometimes the critic is far more concerned about her own image than about the book. Take the "her" as a generic, not a specific sex. So she knows what she 'should' like and dislike, and goes for the books that don't fit that. Maybe a genre like romance, maybe some kinds of sex, anything. Or she simply wants to show everybody how clever she is. Gah.
But sometimes, when you read a book, you want to tell the author what specifically it was you didn't like. Note, you're not saying you don't like the author, or even that you don't like her other books, but perhaps there was something you didn't like, or you felt let down in some way.
It can help an author get better if you say something, but you don't. Because that author might decide that you're her enemy and go for the throat. Or may become so vociferous that you wished you'd never opened your fool mouth in the first place.
I'm very busy these days, doing my best to write and improve, and very happy. But I still have time to read. And when I read something I don't like where do I go? To about 3 or 4 friends I can really trust not to tell anyone or pass anything on.