An old handicraft
An authentic way to connect a book cover with the book block can be obtained by “sewing” (singer sewing). You can choose the spot yourself, right through book cover and book block. This can be done directly on the spine, or you can decide to have more distance from the spine (the book opens less well, but that can be a choice). You can play with the color of the sewing thread, all color combinations are possible, both above and below. There might be a risk that the thread will come loose (in all fairness), even if you choose to let the ends “melt” together. On the other hand… this effect will supply the book a more traditional look.