when a book is at rest on a table, there are 2 equal forces acting on the book but in opposite directions, the table exerts an upward force, the normal force, that pushes on the book the upward force is equal to the weight or the gravity acting on the book, the 2 forces acting on the book are equal but in opposite directions, thus the resultant force is zero and the book is stationary, as the book is at rest, acceleration is zero
