From “Modern Bookbinding.”
Hold the book with its back on a smooth or covered table; let the front board down, then the other, holding the leaves in one hand while you open a few leaves at the back, then a few the front, and so on, alternately opening back and front, gently pressing open the sections till you reach the center of the volume. Do these two or three times and you will obtain the best results. Open the volume violently or carelessly in any one place and you will likely break the back and cause a start in the leaves. Never force the back of the book.
A connoisseur many years ago, an excellent customer of mine, who thought he knew perfectly how to handle books, came into my office when I had an expensive binding just brought from the bindery ready to be sent home; he, before my eyes, took hold of the volume and tightly holding the leaves in each hand, instead of allowing them free play, violently opened it in the center and explaimed: “How beautifully your bindings open!” I almost fainted. He had broken the back of the volume and it had to be rebound.