it could be interesting to monitor the gradual disappearance of physical copies of books by (for example) monitoring the frequency of listings of that book on ebay, or the amazon marketplace (based on the assumption that books that perish are not re-listed, although I'll admit that books are 'sticky' and could bounce around owners until settling with owners who are keenest to hold on to them long term, so you'd perhaps expect decreasing market volume/listing frequency even without any perishing)