I like finding interesting things in the spines of old books. Of course, after sewing a textblock there is often paper glued to the spine and it is entirely invisible once the book has been cased. I've taken workshops over the years where much fuss is made about what to use for spine linings and how some types of paper are perfect for this purpose and others are not. That is certainly true sometimes and hand binders can take the time to think about that nowadays, I suppose. Though, evidently, in commercial binderies a hundred years ago, when full editions of books were being made by hand, they used whatever paper they had for the spine linings and it was not unusual to use offcuts or scraps from the bin. So, we often find printed material in there when we're repairing books now. A few lines from The Winter's Tale, why not!