... a cel that has both a foreground and a 
background. The foreground pretty much looks like a 
regular piece of background (watercolor on paper?) 
except that it's in front of part of the cel. My 
Gundam Wing cel below can illustrate...




The cel is oversized so my scanner cut quite a bit 
of it off, but it's my only book cel. ^^; Anyway, 
you can see the regular background of sunset sky 
and clouds, but the airport runway is actually on 
paper in *front* of the cel. I guess it adds more 
of a 3D effect to the scene.

I would think that book cels with all their 
components would be worth more (just as cels w/ 
original bgs are generally worth more then ones 
without) all depending upon the series and scene, 
of course.