Here is what I read in Brian’s guide to anime 
cels (link on the left at cels.org) : ‘A cel with 
the words book on the top means that there is a 
foreground that is supposed to go in front of the 
cel. This foreground is usually made of the same 
paper as the background, but is cut to the exact 
shape of the item that is to be placed in this 
foreground.’ 
I just received my first book cel, from X. And it 
doesn’t correspond to the definition at all : the 
layer on which it’s written ‘book’ is a cel layer 
(not made of paper) , and it’s not even on top of 
the other layers : the character (Fuuma) has its 
own layer, and this layer is on top of the ‘book’ 
one. So is it an exception, or is this definition 
totally wrong ??