This came in my Daily Dose Steep and Cheap email.

If you don't like a book someone recommended, keep it to yourself. They won't take it well when you tell them it was bad, and they'll be especially upset if you launch into a well-reasoned explanation supported with literary precedent. I learned this not when I compared the Poisonwood Bible to a doorstop, but when the friend who had recommended the Poisonwood Bible responded with a dismissive shrug when I asked what he thought of a book I'd loaned him. Books take so much of an investment in time that I think they form a bit of a cognitive dissonance for their readers. Unless you quit reading halfway through, it's like you've invested so much time in the book that it had better be good. Taking a particular liking to a book is similar to finding a song you enjoy deeply--there's something about it that resonates with you. So when someone comes back with a negative review of a book you like, it's more like it informs your differences with the person than your taste in books. And that person can never be trusted again.