Within a book on a desk, there are billions of forces pushing and pulling on all the molecules. why is it these forces never by chance add up to a net force in one direction, causing the book to accelerate "spontaneously" across the desk?

For exactly the same reason that all the molecules of air in the room, each one
moving at a random Gaussian-distributed speed in a random direction, never by
chance all wind up on one side of the room at the same time, leaving you to
suffocate with no air on the side where you are.

By the way, there are not billions of forces pushing and pulling on all the
molecules in the book, but we know what you mean.   

The answer is in the behavior of large numbers of 'randomly distributed' things.
The behavior of any one of them is unpredictable, and completely up to chance
and the tea leaves.  But the laws of Physics that lean on every molecule result
in a definite probability of how large numbers of them will behave. 

The probability of the book levitating off the desk, or all the air in the room
moving to one side, is NOT zero.  There is a definite probability, which can be
calculated, that either of those events will happen.  But the number is so small
that it can safely be assumed to be zero, without significantly changing our
concept of what to expect in our daily life.

It's NOT correct to say that it CAN'T ever happen, but it's totally rational,
reasonable, pragmatic, and practical to live our lives as if it can never happen,
because the probability is so small.