I suspect not many people go and buy religions texts. Most people seem to get them for free or as a gift, so I'll skip that.

Dictionaries and reference books like encyclopædia don't get read much, but that feels like cheating, because that's not really what they are for.

I'd guess something from classic children's literature? I bet a lot of adults have never read Robinson Crusoe but buy it for kids. Or they pass on the copy that someone bought them as kids, that they never read. As a kid I managed to get through some classic literature, but I'd sometimes encounter one that was actually less interesting than just... doing nothing and waiting for time to pass.

As an aside, I don't think there's anything wrong with having books around that you haven't read! It seems most of the value of a library is in the books you haven't read yet. Or refer to, without fully reading, to inspire you as you need. Or even just have because you think they are interesting or contain ideas of value, and hope to get to someday. The books I've actually read just get shoved in boxes somewhere dark and dusty. On my shelves or on display are all the things I haven't gotten to yet!