Yesterday I had the enormous privilege of a visit to the old library at St John's College , Oxford. As readers of the late, great Terry Pratchett will know, books have to be chained up to keep readers safe.... ...dangerous things books. Chained books had to shelved with the spine facing away, so the shelf mark had to be written on the leaves of the book. A rod would have been passed through the ring on the end of the chain.... ...and then secured in this gap between the desk and the bookshelf.