Q.Physics textbooks are stacked one upon another on a table, one volume above another.

A free body diagram (FBD) isolates one chosen object and shows only the forces acting on it — never the forces it exerts on other bodies. The systematic procedure: (1) identify every force acting on the object; (2) represent the object as a single point; (3) draw each force as a vector arrow from that point, in its correct direction and (where relevant) relative magnitude; (4) apply Newton's second law along each convenient axis.

The single most important — and most often violated — rule is that forces the object exerts on other bodies must never appear in its own FBD. For example, a book resting on a table has exactly two forces in its FBD (weight $mg$ down, normal force $N$ up); the (Newton's-third-law) force the book exerts back on the table, and the force the book exerts (upward) on the Earth via gravity, belong in other objects' FBDs, never the book's own. …