Each book I make is unique on its own right - they are like children to me. I spend some "pregnant" time with each one, until the idea formulates, grows and is ready to be born. Then I start touching the newborn, shaping it, cuddling it, train it to have all the necessary qualifications to become a book. Sometimes with a soft hand (you should watch me when I prepare and apply the inks), sometimes with a hard hand (I use a hammer to shape or round the block), sometimes with trembling hand (especially when I am "compelled" to add another layer of paint, although I am almost satisfied with the cover- not knowing if I will "destroy" it or not. Keyword here is "almost").
Most of the times, just before the book reaches its adulthood- means after the book block and cover are ready but not attached- it needs a caress, to be able to go on while being forever linked with its creator. So I mark its spine. Sometimes with a song, sometimes with a quote, sometimes with just a wishing well. No one will be able to see it, until the book falls apart (and I make sure that THIS will happen after some lifetimes), and hopefully means so much by then, that her owner applies to a restorator to shape her back. But it is there, it is my strength spell and it will keep whispering my words to my book for ever, no matter how many new bonds she will make.
Many bookbinders do that. So next time you hold a handmade book , you will know. It is like a child to someone and it is bound and kept together by love.