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Harry is viewing the situation from an inertial reference frame that
was commoving with the lab frame at time 0.
He's the one that tells
us that there is no gravitational force.
He's the one that tells us
that the reason we don't notice the book gaining momentum and
velocity is because we are riding along right beside the book--our
velocity and momentum are changing too.
He claims that there is no
gravitational force (and hence we shouldn't concern ourselves with
upward flow of downward momentum)
and advises us that if we want to
determine whether or not some apparent force is an actual force in
accord with Newton's third law, we have to look at the situation from
an inertial reference frame.