A railway wagon of mass 15 tonne travelling at 9 km/h along a level track receives an impulse of 5 x 10 kgm/s in the same direction as its own velocity.

Calculate the velocity of the wagon immediately after the impulse.

There's an easy way to tell whether the forces on the book are in balance,
and we don't even need to know how much it weighs.

You said the book is "RESTING".  Right there, it tells us the whole story.

Newton's 2nd Law of Motion says that if the forces on anything are
NOT balanced, then the thing is accelerating ... it's either speeding up,
or slowing down, or else changing its direction of motion.

The book is not doing any of these things.  So the forces on it
can NOT be unbalanced.  Therefore they must be balanced.