I'd like to tag on a request for even more info! I've seen
some cels listed as "book cels" but I don't have a CLUE what
it means, and why the term "book" is used (well, maybe I do 
have a clue from oion's post, but not much of one) And how
can a cel have a paper foreground?? Or does it mean that the
background isn't paper either??

Az, totally confused, and glad the topic came up.