why [and when] bookcloth?
When a book has a covered spine, the areas between that spine and each cover board are getting pulled back and forth whenever the book opens and closes. Fabric is more flexible and durable in motion than paper is, and so bookcloth is the perfect material to bridge those gaps. If you wanted to attach a plain piece of fabric to a book, glue would soak right through the fibers of that fabric to the other side, making a mess and ruining the visual of the fabric. Because bookcloth is backed with paper, glue can be applied without seeping through, and the strength of the fabric can still be harnessed.
Many people enjoy the fabric texture that bookcloth adds to hardbound books, and it can be used to cover your book in its entirety if desired. Or, like in the example above, bookcloth can be applied just over the spine and partially onto the front and back covers, leaving the remainder of the book open to be covered by decorative paper. Additionally, bookcloth can be used for purely aesthetic reasons with open-spine books where it’s not at all necessary for spinal support and it just looks and feels good.