Sorry to be confusing.  A "book layer" is a non-
cel layer meant to go over top of a cel.  
There's another layer that goes underneath the 
cel, which then is enclosed on both sides like 
the pages between the front and back cover of a 
book (hence the name).

The production book layers I have are made of a 
heavy-duty watercolor-type art paper and are hand 
colored, not photocopied.  They are usually taped 
directly to the front of the cel, using that 
toxic Japanese adhesive tape that yellows and 
dries up.

You can see an example (with a closeup of how it 
looks) at 
http://sensei.rubberslug.com/gallery/inv_info.asp?
ItemID=32826

But from what you say, you've got a low-quality 
background replacement.  I have both original 
(watercolor) and laser-copy backgrounds, and 
they're both pretty good, though obviously the 
originals are nicer.  I have seen on dealer and 
auction sites sets that have characters as well 
as backgrounds on these bottom laser-copy layers, 
and usually this is made clear in the description.

My guess is that someone early on obtained the 
entire series that included your cel, kept the 
nicest one with the bg and/or book layer and made 
color photocopies of the setup to help sell the 
others.  This someone may not have been the 
person who sold you the cel, however.  You're 
justified in returning it as not-as-described, 
however.