Book cels usually refer to a full setup of multiple layers including backgrounds. The setup will include the main background, the characters or character involved in the scene, and then another layer that is actually part of the background. The example below shows the background including the trees, blue sky, and building, two separate layers of the characters Sakura and Yukito, and finally notice how the bench arm is in front of Sakura's arm that is the top background layer. This setup helps to give 2D animation the illusion of depth.
Weird JD Note: I never thought book made much sense I always personally called them sandwich cels cause you have two layers of background around a nice cel center. ^_^;;
|Book Cel from Cardcaptor Sakura the Movie 2 - Sakura and Yukito