I'll go either way.
I'll nom a book I've read and admired, or that I think would be interesting to discuss (I would feel obliged to reread it if it won the vote).
Or I'll also nominate a book I haven't read, but that I would like to at some point.
While I'm here, I'll nominate "A History of Western Philosophy" by Bertrand Russell. I have read it before, but would happily read it again.