Using your hand to exert a horizontal force, you push a physics textbook across the floor at a steady pace. The frictional force exerted by the floor on the book opposes its motion. Is the "push" force exerted by your hand greater than, less than, or equal to that frictional force?

It's equal to  the friction force.

Explanation:

When moving at a constant speed, the book must obey (as any body) Newton's 2nd Law.As a consequence of its mathematical expression (F=m*a), if a=0, this means that the total force must be zero.If we decompose this force in two components perpendicular each other (horizontal and vertical, for instance), both components must be zero too.Taking the horizontal direction, there are only two forces acting on the book: the pushing force exerted by the hand, and the friction force oppposing to this force.Both forces must be equal and opposite each other, in order to the total force be zero, and the book can move at a constant speed, as requested.