Scenario 1: It’s a book so wonderful that all the critics and reviewers are falling over themselves to praise it to the skies, it’s been shortlisted for several awards, and your highbrow cousin who teaches college literature can’t recommend it highly enough.
Scenario 2: Everyone is reading it, it has a budding cult following and a Hollywood movie is in the making. 6 people have recommended it to you already because oh-my-God it’s their favourite book in the whole world!, and it’s been on top of the best-seller lists for months.
Outcome: You acquire a copy of either book, open it and by page three you’re wondering what everyone sees in it. Because you don’t like it. Not after 3 pages, not after 20, not even after the last page is turned. The reasons vary. It may be the writing style, or the story, or that indefinable something that makes a book come alive for you, but you just couldn’t get into it. It might be because you found it boring, or because you thought it was trying too hard, or the characters were unsympathetic, soulless or flat (or all three), or it was drowning you in a flood of words. The story might be good but the writing atrocious, or the writing may be lovely but the story-line may reveal serious lack of research, or it may be over-researched with more back-story than actual plot. It might be any of a hundred other things, but the end result is the same: you think it’s not that great or even good a book.
Saying so about book 1 is going to make you look like an ignorant pleb who doesn’t understand Literature. Saying so about book 2 will antagonise the fans and make you look like a literary snob.
Believe me, I’ve been there too. If you really do care what people think about your reading likes and dislikes, the best solution I can offer for the first dilemma is to be able to put into (preferably three+ syllable) words why you didn’t like it, giving examples and substantiating your opinion.
As to the second instance, a dedicated fan of, say, Twilight or The Lord of the Rings, is not going to be amenable to listening to an analysis of what you see as the novel’s faults, but saying it’s a matter of taste is generally a safe bet.
Then there is the cop-out: listen to what the other person thinks about the book, and then agree with them. This is the best way if you are trying to impress someone who is not likely to be impressed by your real opinion, however well articulated, but you had better be careful not to accidentally reveal your real opinion or to agree too effusively, or you will be labelled a sycophant.
But – and this is what I really recommend – the very best way is to develop a thick skin. If a person is shallow enough to judge you on the basis of what kinds of books you like or don’t like, it’s their problem and not yours. If it still bothers you, you can always fall back on the fact that you can’t possibly be the only person who didn’t like that particular book, and where better to find those others than on the Web?