At this point, let's explain the term "Bleed". The sheet
of paper that goes through the printing press is always larger than
the actual page size of the final book. The final printed sheet is
later folded and cut to size. This is the trim size. "Bleed"
refers to the process of extending any image that comes to the edge
of the trim and adding 1/8 inch more image. The reason for adding
"Bleed" is that a sheet of paper going through the printing
press can bounce slightly against the many "guides" used
to keep it going straight through the press. If you didn't incorporate
bleed, a sliver of white could show on some pieces when your cover
is finally folded and trimmed. Note that "Bleed" must be
added not only to your cover, but also to any pages where an image
goes right up to the trim.