Could someone possibly tell me a little more about book cels?

I have not been able to find much information on them, except
that the cut-to-shape foreground is usually made from the 
same material as the painted background. I also want to know-
and this sounds like silly question - if book cels ALWAYS has
"book" written on them. This is because I have a cel which 
has a cut foreground (which I think is made from the same
paper as the background) stapled to the front of the cel, but
"book" isn't written on it. >_^' I'm assuming there are some
other requirements to make a book cel, but I don't know them.
-_-'

Thanks!