Interesting suggestion, though it'd be interesting to see how that varies across titles with copies.
A book selling one copy which is never read would have a higher percentage-unread.
Otherwise, as total copies increase, the absolute magnitude of unread copies would likely increase proportionately.
Keep in mind that copies presented to multiple potential readers (as with, say, Gidion's Bible, typically found in hotel rooms) would likely have a greater chance of being opened at least once over its lifetime.
There's also the interesting question suggested: what book has the highest readers-per-copy ratio? And how should that be scaled by total readers? I'm inclined on general principle to suggest the ratio * log(n) where n is total copies produced. That penalises very low sales volumes, but discounts greater total print runs.
I'd suspect it would probably be a romance novel.
