lovcrimson said:
BOOK = foreground objects
It is substitute for 'reg line'. (In Japanese, reg line is 'Kumi クミ') 'Book' background can be placed on or beneath the cel, and is able to slide freely. In cel era, term 'book' meant to paint the background on cel, or to cut out part of the background and attach them to cel. Now in digital compositing, compositors cut out the 'book' background in compositing or background painter paint the book on separate layer.

There are terms related with book: Sliding multiple book backgrounds in different speed is called 'Attached multiplane' (密着マルチ)
When camera pans, sliding the cel or book backgrounds in opposite direction is called 'Attached slide' (密着SL)