A signature is often made from a large sheet of paper with 16 pages printed on each side, then repeatedly folded to create a 32-page signature. This method of construction, which is standard throughout the publishing industry, is why you will sometimes see books with several blank pages at the front or back The final signature contains 32 pages, but the text might fill only 29. Each of these signatures is later attached to the spine to create the book.
Once the signatures are assembled, the next step in bookbinding is to attach them to the spine. Traditionally, the signatures on a hardcover book were stitched to the spine with sturdy thread, so that a book would lie open if placed on its spine. If you have any older books, you might see that the signatures seem loose and are not attached to the cover. In more recent hardcover books, the stitched spine is glued to the cover.
After the signatures are attached to the spine, the pages will stick out unevenly on the edge of the book. At this point, a bookmaker can choose to leave the pages rough and uneven, or trim them for a uniform length and feel. Traditionally, this trimming was accomplished with tools called "ploughs," which are still available today. The groups of signatures are tightly held together so that they do not shift while they are being trimmed.
The final step in the binding process for hardcover books is the attachment of the cover itself, which includes a board for the front and back, as well as one for the spine. This can be covered in leather or fabric, though it is sometimes simply coated with paper. There are a number of ways to attach the cover, but a common technique involves attaching a small piece of canvas to the bound signatures and then gluing it to the inside cover.