There are books that I have read that I do not like. There are books that I have started, and disliked so much I have stopped reading them. There are books that I have looked at in the bookstore, and then placed back on the shelf. There are books that I walk past, never even making eye contact with.
I think all of this is fine. A wide variety of books means a greater likelihood of there being something for everyone. A set of readers with a wide variety of literary tastes means a greater likelihood that there are people out there who will like what I write.
But I will almost never tell you when I don't like a book.
The primary reason for this being that I am a writer, not a book reviewer. My job is not to tell you my opinions on books, but to write books and hope that you will one day have an opinion on them. When I do mention a book here, it's because I have enjoyed reading it and I want to share that experience with you.
The secondary reason is, that sometimes, when I don't like something, it has nothing to do with the quality of the writing. It might just be that it wasn't my sort of thing. (To draw an example from outside the world of books, I never made it through more than about half of the first season of Battlestar Galactica. I thought it was well-written, well-acted, and it depressed the hell out of me. And yes, I can hear them coming to take away my geek card even now.) When I was in grad school, I took a seminar where we had to review certain of the texts we studied that semester. My professor told us to be sure to review the book that the author had actually written, rather than the book we wish the author had written. I thought that was really good advice, and I still adhere to it. So if my enjoyment of a book was lessened because what I really wanted was a sexy vampire, and the author gave me a grumpy zombie instead, well, that's on me, not on the book, and I don't need to punish the author for that with a bad review.
There may someday be a set of circumstances where a book goes so horribly wrong, or pisses me off to the extent where I will need to rant a bit and tell you why. But I'll leave that discussion for that day.