By 'lower edge' I mean the edge that rests against the bench.
As the book lies open, there's a lovely shadow between the spine and the cover. In this video I create this shadow and shape the outside of the spine. I don't carve all the way from one end of the book to the other - I'm assuming no one will look! - but you certainly can if you wish.
Take your time to get this lower edge right; it forms a guide to both the inner edge and hollow ends of the spine.