As previously noted, Book cels (and douga/genga) 
have been discussed in archived threads and are 
in a couple cel faqs.  I'm going to try to 
briefly explain but I encourage also reading 
through those threads for a better idea through 
different wordings and points of view.

Book cels are usually cels with multilayer 
backgrounds (whether this means two layers of 
background behind the cel or a background layer 
and a foreground layer or 'more').  People like 
to describe it as a sandwiching of cels between 
foreground and background, as if the paper stock 
were bookends.  I've seen 'book' written on some 
background or foreground layers (though sadly 
sometimes the corresponding layer is missing).

Looking at genga and douga, douga should look 
much more simple (and perhaps more 'clean').  
Douga are used to generate the black lines on 
cels (or now CG I guess ^_^).  Genga will not 
exactly match a cel.  Also, the sequence number 
on a douga will usually be in the upper right 
hand corner or somewhere along the edge of the 
paper.  The sequence number on a genga will 
usually be somewhere more towards the middle, not 
on the edges.  Hope this helps!

Keys