“What proportion of the books you own are unread?”
I’d say that perhaps 20-25% or so of my books are of the “reference” kind rather than the kind you read straight through from cover to cover. Besides dictionaries and grammar books etc I have quite a few on for example history, religion and mythology that I have only used to look things up in - even if also some in those genres that I have read from cover to cover as study course books.
When it comes to novels, I don’t have many of those unread. Except on my Kindle… where I’ve collected lots of free ones in the last few years.