From The Book on the Bookshelf, Henry Petroski:

A common procedure, when there is room of course, is to put one's finger on the top of a book and pull gently against the headband to rotate the book in its place until its top corner projects out enough from the other books on the shelf for it to be grapsped and removed. Martha Stewart Living does not approve: "Never hook your finger over the top of the spine." The problem with doing so is that it can lead to broken fingernails or, perhaps worse, to torn book bindings. As a nineteenth-century "handy-book" warned, "Never pull a book from the shelf by the head-band; do not toast them over the fire, or sit on them, for 'Books are kind friends, we benefit by their advice, and they reveal no confidences.'""