But those things aren't the answer to OP's question, are they? I'm sure that out of all the Harry Potter or DaVinci's Code or whatever whatever popular book you look at there'll be a nice % of books that haven't been read, but I'm pretty sure that a majority of.peoole that buy them also end up reading them.
The more reasonable answer would probably be something that's popular but not necessarily something you read. Like others have said, a dictionary, cookbook, or book related to some other skill. Those are a lot more likely to go unread
I get it. I didn't imagine books could be purely ornamental, but the consensus seems to be that's not too rare. I personally organise them broadly by genre and theme, so if I have to recommend something to friend I will point at a particular shelf and go "look in there"
I have a pretty decent sized library. My fiction section is about 95% read, but the non-fiction sections are much less. You sometimes buy non-fiction as reference materials, to flip through, etc. Not necessarily to read cover-to-cover. (I'd guess my non-fiction is 25% read.)
The OP seemed to imply that not reading the book was always the plan, not because it was dropped halfway through because it's boring. But perhaps I'm reading too much into it