Sometimes people will suggest that you read a certain book they happen to own. Of course, they won't just recommend anything, but one of their very favourites. They will offer to lend it to you. Should you accept? Perhaps not. Consider:
1. If you're like me, the baseline probability that you will enjoy a given book (film, song, painting) is low: most stuff isn't that good.
2. What's really interesting is not whether it's good, but whether using your time to read that book (watch that film, etc.) will yield more utility than the best of the other possible uses of your time. And there are a lot of interesting books out there that you haven't read yet.
3. Borrowing a book is not like borrowing a hammer or a car. When people offer to lend you a book, they are asking you for an assessment of their soul. Yes, I'm exaggerating, but only a little.
4. It is impossible to simply return a book with a thanks. Try it one time. You will find that you cannot get out of that situation without an assessment of the book.
5. If you loved it, fine. But there is quite a chance that you didn't (see point 1). What now? You can lie, but that's bad, because (a) you don't want to lie, and (b) in order to spin a convincing lie, you might feel compelled to finish the book first, which you don't want to do if you don't like it. Or you might offer a truthful assessment. But remember what that really is an assessment of.
6. If you go for the truth, on top of hurting people's feelings, you'll subject yourself to their reaction to having their feelings hurt. In the present context, this often comes in the shape of interpreting your not liking the book as a defect of yours: "Oh, it was probably too far out for you, right?" "Oh, now I remember, you did tell me you enjoy John Irving. No surprise, then, I guess." Do you enjoy listening to what a mediocre mind you have? I don't.
All of this suggests you shouldn't borrow books from mere acquaintances. If you think a book they're describing sound intriguing, (i) proactively mention how many books are sitting on your shelves, unread ("If only I had more time!"); (ii) get a copy without letting them know. If you enjoy it, tell them about it; otherwise, never bring the topic up again.
What about borrowing books from friends? Chances are that they'll be cooler about it, as they already know that you have a firmly positive assessment of them, and if they were oh so touchy, you might not have selected them as friends. So, should you borrow books from friends? That's your decision, after all, you know them better than I do. But borrowing books from acquaintances? That seems like a bad idea.