Book cels have background setups that cover the 
front and back of the cel, kinda like a sandwich.

This is an example of one of mine:

http://www-personal.umich.edu/~tdkim/cel/epyon-
gundamW-A1END.html

It's a one layer cel with a background and a 
foreground that covers the left, bottom, and 
right sides of the cel.