What, 50 shades of grey? Hope you’re kidding.
If you are willing to read this kind of immoral stuff, then read it and tell your sincere opinion. If in truth you do not read this, then just tell as nicely as possible that you simply don’t enjoy this kind of literature.
It’s called being polite without actually committing yourself one way or the other on something. It’s difficult to explain, but, basically, if you are concerned that telling the obvious truth about the situation (“Why in the name of all that is holy did you give us a pornographic novel?!”) would adversely affect the relationship to the point where you won’t exactly be able to help such a person change their mind later on in the relationship, then you say something to fill the space but that doesn’t make a positive statement, then change the subject. Technically, it’s true: presumably the gift-giver gave the set of books in the hope that the recipient would enjoy them, so it was a thoughtful gift, however flawed the thinking.
ETA: As I said before, I’d probably go with the last response I posted rather than this one, in any case. Also, I’m firmly in the “burn it” camp–just don’t, obviously, tell the giver that.
Remember, this garbage is seen as “normal” by most of the population. If you don’t know the person all that well, then ranting about how immoral it is might feel good, but won’t win the person to your way of thinking. If, when my husband and I got married, a distant relative or a friend (who I’d never met) of my in-laws gave us a book on marriage which promoted birth control and immoral sexual activities, I probably wouldn’t want to have a detailed discussion of those subjects with someone I didn’t really know. I would utter the proverbial polite nothing (“It was so thoughtful of you to give it to us”) and leave it at that. Courtesy never harmed anyone. The OP has subsequently made it clear, however, that he’s rather closer to this cousin than that, so the point is moot, anyway.
